Depend intgration pipeline execution on branch protection state

Previously we had the master branch hard-coded in a variable in order to
determine whether to run the integration pipeline (on non-master branch)
or the merge pipeline (on master branch). This variable had to be
adapted everytime we copy a branch, which can potentially be forgotten
and lead to problems.

Remove the hard-coded branch and use the branch protection state for
this decision instead in order to make this more flexible.

manifest-integration.yml

@@ -40,15 +40,15 @@ workflow:
     # FIXME: Unfortunately we cannot use variables in regular expressions due to a
     # GitLab limitation: https://gitlab.com/gitlab-org/gitlab/-/issues/209904
     # As soon as this get fixed, use the regex based rules below instead of checking
-    # against the MASTER_BRANCH variable.
+    # if branch is protected.
     # Run pipeline if target branch of the merge request has an integration target, i.e.
     # INTEGRATION contains a line beginning with the target branch followed by a colon.
     # This also implies that the pipeline runs in merge request context only.
     # - if: $INTEGRATION =~ /^$CI_MERGE_REQUEST_TARGET_BRANCH_NAME:/m
-    - if: $CI_MERGE_REQUEST_TARGET_BRANCH_NAME == $MASTER_BRANCH
+    - if: $CI_MERGE_REQUEST_TARGET_BRANCH_PROTECTED
     # Run pipeline on target branch after merging a merge request.
     # - if: $INTEGRATION =~ /^$CI_COMMIT_BRANCH:/m
-    - if: $CI_COMMIT_BRANCH == $MASTER_BRANCH
+    - if: $CI_COMMIT_REF_PROTECTED
 
 .skip-for-gitlab-ci-integrations:
   rules: