Describe the PR

While release, I've seen the following warnings:

[DEPRECATION] Template.register_filter is deprecated. Use Environment#register_filter instead. Called from /home/andreas/PMD/source/pmd/docs/_plugins/custom_filters.rb:136:in `<top (required)>'
[DEPRECATION] Template.register_tag is deprecated. Use Environment#register_tag instead. Called from /home/andreas/PMD/source/pmd/docs/_plugins/details_block.rb:22:in `<top (required)>'

The reason is, that we use in our toplevel Gemfile already liquid 5.x, but in docs/Gemfile we still use liquid 4.x. We also can't just update to liquid 5 everywhere, as jekyll still uses liquid 4.
Furthermore, the pmdtester uses liquid 5 already.

I've cleaned up a bit, how we call render_release_notes.rb, so that we call it within the docs directory. Then we can reuse the Gemfile from docs rather than using a group.

The toplevel Gemfile is then only used for Danger and pmdtester.

Related issues

• Fix #

Ready?

• Added unit tests for fixed bug/feature
• Passing all unit tests
• Complete build ./mvnw clean verify passes (checked automatically by github actions)
• Added (in-code) documentation (if needed)