Use v4

Signed-off-by: Andreas Kern <[email protected]>

.github/workflows/filename-check.yml

@@ -6,7 +6,7 @@ jobs:
   check-filenames:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Check for capital letters in filenames in 'doc/manual'
       run: |
         files_with_caps=$(find doc/manual -type f | grep '[A-Z]')