GHA release workflow supports multiple tags via github-tag-templates input field

[shreyas-s-rao]

GHA release workflow supports multiple tags via github-tag-templates input field. I have replaced the previous github-tag-template field with github-tag-templates, since I nobody is using github-tag-template in gardener org yet, and I wanted to avoid the complexity of handling the same input from multiple input fields and deprecation overhead.

Also, this field is technically a string (comma-separated values validated in release/action.yaml), since the schema does not allow array or list type in reusable workflows - ref discussion.

I also split push release-tag step in release action into push tag-refs and push release-commit for clear separation of responsibilities.

Please let me know if the approach is correct. Also, majority of the changes are AI generated, so please let me know if there's anything I need to correct here as well.

Fixes #1381

Release note:

GHA `release` workflow now supports multiple tags via `github-tag-templates` input field which takes comma-separated values of target ref templates.

GHA `release` workflow input field `github-tag-template` has now been replaced by `github-tag-templates` which takes comma-separated values of target ref templates. While the input name needs to be adapted, no change is needed to the existing value.

[gardener-robot]

@TuanAnh17N, @ccwienk, @zkdev, @8R0WNI3 You have pull request review open invite, please check

[ccwienk]

generally-speaking, it is a reasonable ask to allow creation of multiple tags (thanks for taking the time to raise a pullrequest, rather than just a feature-request :-)).

However, I would prefer this change to focus on the actual feature (one tag -> list of (extra-)tags). Refactoring and adding of validation code may also be adequate. However, I would prefer such changes to be done in separate commits.

Also, with such increased amount of code, I would prefer to mv majority of code into a separate script (any file within an action's directory is available at ${{ github.action_path }}).

[gardener-robot]

@shreyas-s-rao You need rebase this pull request with latest master branch. Please check.

[shreyas-s-rao]

@ccwienk thanks for your suggestions. I have now added a new field github-additional-tag-templates and retained the previous github-tag-template. The new field accepts a newline-separated list of tag templates. PTAL, thanks.