Logstash versioned plugins build failures vs auto-PR and required manual cleanup

With a constant branch name introduced in 114fe574eb12687bf455a5525ac739fdce69ca0d, once the branch exists on `origin`, subsequent failed builds will fail to push their changes to the constant branch, since the push is not a fast-forward of the existing dereference.

This prevents the diff for the existing PR from being updated, meaning that once a build fails attempts to fix the build are outside the PR feedback loop.

Additionally, after a failed-build-PR is merged or closed, users _must_ remember to manually clean up the branch (e.g., by deleting it in the Github UI or by pushing a deletion with `git push origin :versioned_docs_failed_build`), or else subsequent build failures will open up a _new_ PR that points to a _previous_ failure.

Should we add a `--force` to the `git push` command when opening that PR? This would cause the in-flight PR to get updated on each subsequent build failure, but the nature of a force-push could be surprising to anyone attempting to manually fix-up the PR.