Allow gem-release CI/CD component to generate changelog in releases

Description

We started to release gems via gem-release CI/CD component in !74 (merged) and fixed it in !75 (merged).

With that we no longer want to track changes manually in CHANGELOG.md but use git trailers instead.

MR template was inspired by https://gitlab.com/gitlab-org/ruby/gems/gitlab-styles/-/blob/master/.gitlab/merge_request_templates/Release.md

This MR also moves the MR template from UI to git.

Related Issues

Closes #17 (closed)

Suggested version bump

• Major (backwards incompatible changes)
• Minor (backwards compatible changes)
• Patch (API compatible changes)

Checklist

• Tests have been added or updated to cover any changes in behavior
• This does not change the API to consume this library, or a suggested version bump has been provided
• Add git trailer Changelog: <category>
• No new runtime dependencies have been introduced