backport workflow to respond to PR tags

[jodygarnett]

The project makes use of backport tags, but relies on manual cherry picking of commits to prior branches, which can cause problems when branches have diverged. This PR automates the task of doing that cherry-pick, creating a PR of the result if it can be cleanly done.

Uses backport.yml:

Backport is a JavaScript GitHub Action to backport a pull request by simply adding a label to it.

It can backport rebased and merged pull requests with a single commit and squashed and merged pull requests.

Combined with our QA checks this should provide less hands-on work for our developer team.

Checklist

• I have read the contribution guidelines
• Pull request provided for main branch, backports managed with label
• Good housekeeping of code, cleaning up comments, tests, and documentation
• Clean commit history broken into understandable chucks, avoiding big commits with hundreds of files, cautious of reformatting and whitespace changes
• Clean commit messages, longer verbose messages are encouraged
• API Changes are identified in commit messages
• Testing provided for features or enhancements using automatic tests
• User documentation provided for new features or enhancements in manual
• Build documentation provided for development instructions in README.md files
• Library management using pom.xml dependency management. Update build documentation with intended library use and library tutorials or documentation

[jodygarnett]

This can be applied to the codebase and tested with #7559