Integrate gitlab-ci/fix-branch-protection-check

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@23680f48

Fix check for branch protection state

Since the changes of 939ade2b9f98b3e1569332270a7ab16ce7d70b4a we observe
that pipelines are executed on non-protected branches, which is not what
we want.

GitLab's documentation was a bit unclear on the according CI variables
CI_COMMIT_REF_PROTECTED and CI_MERGE_REQUEST_TARGET_BRANCH_PROTECTED, as
stated in this stack overflow answer:
https://stackoverflow.com/a/59023344/3018229

We need to explicitly check for the variable values to be "true" instead
of just checking whether they are set.