prepare release action: Make version input optional

The idea is to run the action twice. The first time without a version
input. It will generate a CHANGELOG.md update and raises a PR. Then the
changelog can be reviewed. based on the categories, the correct new
version can be determined and the action can be executed again.

.github/workflows/prepare_release.yml

@@ -6,7 +6,7 @@ on:
     inputs:
       version:
         description: 'Module version to be released. Must be a valid semver string without leading v. (1.2.3)'
-        required: true
+        required: false
 
 jobs:
   release_prep: