Prune old branches

[maxime-rainville]

We have a lot of old branches which makes it harder to manage where changes should go.

Acceptance criteria

• We establish a process for deleting old branches on all repos.
• The process is intertwine with our release process.
• The process handles old PR targeted at branches that are about to be prune.
• When the process is followed, only branches for supported releases are retained.
• The process accounts for exceptions (e.g.: LinkField has some older "unsupported" branches that are still under active development)
• Process can be automated.
• Process is documented on the GitHub repository management page.