Delete stale branches

The https://github.com/apache/maven-surefire/branches/stale page lists 25 stale branches, half of them being at least 3 years old. I suspect that the `mvn4` branch, for example, is not applicable anymore since Maven 4 got many changes in the last years. Does someone familiar with those branches could delete the obsolete ones?

Review the repository's stale branches page and inspect the listed branches, starting with `mvn4` and its relevance to the current Maven Surefire project. Confirm which branches are obsolete with the issue discussion or project history, then delete only the branches that are no longer needed.

Written by the indexing model from the issue text.