Add License Check + Consolidate all workflows

[VJanKraemer]

Subject

Add the missing license check workflow + consolidate all workflows into one, better structured workflow. This also means, that the UI/Display of the CI checks is nicer than before!

Description

No Issue/Ticket available

Instructions for review / testing

Tested here: https://github.com/VJanKraemer/sil-kit-ci/pull/2

Developer checklist (address before review)

• [ ] Changelog.md updated
• [ ] Prepared update for depending repositories
• [ ] Documentation updated (public API changes only)
• [ ] API docstrings updated (public API changes only)
• [ ] Rebase → commit history clean
• [ ] Squash and merge → proper PR title

[VJanKraemer]

We need to fix the required workflows in Github before we can merge this PR @MariusBgm