schema test coverage: set exit code only if base branch is not dev

[ralfhandl]

Only report schema test coverage below 100% as a failed check if the PR base branch (==target) is not dev because we know that dev doesn't have full test coverage and still want the other tests as required checks.

Tested with PRs

• Dummy 3.2 #4714 targets v3.2-dev, has full test coverage, all checks green
• Dummy dev #4713 targets dev, has lacking test coverage, still all checks green

TODO

• close dummy PRs after merging this PR

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• schema changes are included in this pull request
• schema changes are needed for this pull request but not done yet
• no schema changes are needed for this pull request