Enable CI for every pull request (no matter which branch)

[nilsreichardt]

Doesn't it make more sense to run our CI for every branch? Because otherwise the CI will not run when we split a PR into multiple branches like:

main
   big_feature_1
      sub_feature_for_feature_1
      sub_feature_for_feature_2

I think it would be helpful to also run the CI when you merge sub_feature_for_feature_1 into big_feature. Of course, there could be the case sub_feature_for_feature_1 has some unfinished tests or wrong lints, because it's part of big_feature, but I think in these special cases we could skip the CI (or using skip: true in the tests).

[github-actions]

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