Extract forward compatiblity checks and run on nightly schedule (#15437)

Running compatibility checks with every Crystal release between the oldest forward-compatible (1.0.0) and latest is quite expensive. It currently takes 9:20 hours (14 * 0:35).
And they're not particularly useful. Checks for each individual release would just help discover an odd compatibility issue that only exists on a specific Crystal version. That's is not very common. In case there are any compatibility issues, they'll most likely affect the oldest release (1.0.0) as well. Testing for that should suffice to identify the vast majority of issues.
So we're pulling them out of the pre-merge checks and only run them on a schedule on master (or on demand).
This helps reduce unnecessary CI usage.

.github/workflows/forward-compatibility.yml

@@ -0,0 +1,64 @@
+name: Forward Compatibility
+
+on:
+  push:
+    paths:
+      - .github/workflows/forward-compatibility.yml
+  schedule:
+  - cron: '0 3 * * *'
+  workflow_dispatch:
+
+permissions: {}
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event_name }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/master' }}
+
+env:
+  TRAVIS_OS_NAME: linux
+  SPEC_SPLIT_DOTS: 160
+
+jobs:
+  # Test against *each* supported Crystal version for forward compatibility.
+  #
+  # This workflow runs on a nightly schedule on `master`.
+  # It can also be dispatched manually when necessary.
+  #
+  # We run a basic version of this test on every commit which covers only the earliest and latest supported versions in `linux.yml#x86_64-gnu-test`.
+  x86_64-gnu-test-forward_compatibility:
+    env:
+      ARCH: x86_64
+      ARCH_CMD: linux64
+      DOCKER_TEST_PREFIX: crystallang/crystal:${{ matrix.crystal_bootstrap_version }}
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        crystal_bootstrap_version: [1.7.3, 1.8.2, 1.9.2, 1.10.1, 1.11.2, 1.12.2, 1.13.3, 1.14.1]
+        flags: [""]
+        include:
+          # libffi is only available starting from the 1.2.2 build images
+          - crystal_bootstrap_version: 1.1.1
+            flags: "FLAGS=-Dwithout_ffi USE_PCRE1=true"
+          - crystal_bootstrap_version: 1.2.2
+            flags: "USE_PCRE1=true"
+          - crystal_bootstrap_version: 1.3.2
+            flags: "USE_PCRE1=true"
+          - crystal_bootstrap_version: 1.4.1
+            flags: "USE_PCRE1=true"
+          - crystal_bootstrap_version: 1.5.1
+            flags: "USE_PCRE1=true"
+          - crystal_bootstrap_version: 1.6.2
+            flags: "USE_PCRE1=true"
+    steps:
+      - name: Download Crystal source
+        uses: actions/checkout@v4
+
+      - name: Prepare System
+        run: bin/ci prepare_system
+
+      - name: Prepare Build
+        run: bin/ci prepare_build
+
+      - name: Test
+        run: ${{ matrix.flags }} bin/ci build

.github/workflows/linux.yml

@@ -13,6 +13,10 @@ env:
   SPEC_SPLIT_DOTS: 160
 
 jobs:
+  # Test against latest release and the earliest supported Crystal version for forward compatibility.
+  #
+  # A more advanced test which runs against *each* supported version is available in `forward-compatibility.yml` and
+  # runs regularly on a nightly schedule.
   x86_64-gnu-test:
     env:
       ARCH: x86_64
@@ -22,24 +26,12 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        crystal_bootstrap_version: [1.7.3, 1.8.2, 1.9.2, 1.10.1, 1.11.2, 1.12.2, 1.13.3, 1.14.1, 1.15.1]
         flags: [""]
         include:
           # libffi is only available starting from the 1.2.2 build images
           - crystal_bootstrap_version: 1.0.0
             flags: "FLAGS=-Dwithout_ffi USE_PCRE1=true"
-          - crystal_bootstrap_version: 1.1.1
-            flags: "FLAGS=-Dwithout_ffi USE_PCRE1=true"
-          - crystal_bootstrap_version: 1.2.2
-            flags: "USE_PCRE1=true"
-          - crystal_bootstrap_version: 1.3.2
-            flags: "USE_PCRE1=true"
-          - crystal_bootstrap_version: 1.4.1
-            flags: "USE_PCRE1=true"
-          - crystal_bootstrap_version: 1.5.1
-            flags: "USE_PCRE1=true"
-          - crystal_bootstrap_version: 1.6.2
-            flags: "USE_PCRE1=true"
+          - crystal_bootstrap_version: 1.15.1 # LATEST RELEASE
     steps:
       - name: Download Crystal source
         uses: actions/checkout@v4

scripts/release-update.sh

@@ -34,7 +34,9 @@ sed -i -E "s|crystal-[0-9.]+-[0-9]|crystal-$CRYSTAL_VERSION-1|g" bin/ci
 
 # Edit .github/workflows/*.yml to point to docker image
 # Update the patch version of the latest entry if same minor version to have only one item per minor version
-sed -i -E "/crystal_bootstrap_version:/ s/(, ${CRYSTAL_VERSION%.*}\.[0-9]*)?\]\$/, $CRYSTAL_VERSION]/" .github/workflows/linux.yml
+previous_release=$(grep -o -P '(?<=crystal_bootstrap_version: ).*(?= # LATEST RELEASE)' .github/workflows/linux.yml)
+sed -i -E "s/crystal_bootstrap_version: .+ # LATEST RELEASE/crystal_bootstrap_version: $CRYSTAL_VERSION # LATEST RELEASE/" .github/workflows/linux.yml
+sed -i -E "/crystal_bootstrap_version:/ s/(, ${previous_release%.*}\.[0-9]*)?\]\$/, $previous_release]/" .github/workflows/forward-compatibility.yml
 sed -i -E "s|crystallang/crystal:[0-9.]+|crystallang/crystal:$CRYSTAL_VERSION|g" .github/workflows/*.yml
 
 # Edit .github/workflows/*.yml to update version for install-crystal action