Split Java code style checks

[koppor]

It is hard for newcomers to know about that we have more than checkstyle in place. See #10496 (comment).

We (currently) don't have it in the documentation. This is future work.

I think, it is more helpful, if we automatically comment on the PR to support tailored guidance. This PR tries to do it.

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[github-actions]

The build for this PR is no longer available. Please visit https://builds.jabref.org/main/ for the latest build.

[koppor]

Build hangs, because I renamed the tests (to be more fine grained).

Will merge and then adapt our CI requirements. Thus, all PRs need to merge main.

[koppor]

For reference, new branch protection rules:

[koppor]

For reference, this is the output if something is really wrong - source: #10571 (comment)