Add GitHub action to enforce do not merge label

[hendrikmakait]

We have the convention not to merge PRs flagged with DNM in the title. This PR makes this convention explicit by prohibiting merges of PRs that are labeled with the new do not merge label.

As an aside: We may want to add a repository for shared actions between multiple repositories to avoid copy+pasting code (https://bgenc.net/2023.02.18.github-actions-do-not-merge-label/).

[hendrikmakait]

To enforce the new action, we will have to require this workflow in the branch protection rule.

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    27 files  ± 0      27 suites  ±0   10h 18m 47s ⏱️ + 25m 29s
 3 996 tests ± 0   3 883 ✅  -  1    110 💤 ±0  3 ❌ +1 
50 254 runs  +18  47 956 ✅ +25  2 295 💤  - 6  3 ❌  - 1 

For more details on these failures, see this check.

Results for commit a4ff845. ± Comparison against base commit cbf939c.

[fjetter]

Did anything happen that we now require this kind of protection rule?

[hendrikmakait]

Did anything happen that we now require this kind of protection rule?

Not yet, but I'd like to keep it that way. I find just having DNM in the title unreliable and default to keeping DNM PRs in draft, but that signals that they're not ready for review. I figured it's a pretty low-effort change that makes things a bit nicer.