Post releases chores

[julienrbrt]

We've released v29. This issue tracks the post release chores to discuss and do.

• v29 has been tagged from main, create release branch
• setup mergify + github labels
• update maintenance policy to explain v28 is in bug fix only, and v29 full maintained version

Given we have no breaking changes planned, we could alternatively wait until v0.54 or any other breaking version of the Cosmos stack to do the above. And keep tagging v29 from main, until we branch off main for its release branch. It may simplify the process for a bit, as most of things that will lend will be non breaking and backportable.

[julienrbrt]

We could alternatively wait until v0.54 or any other breaking version of the Cosmos stack to do the above. And keep tagging v29 from main, until we branch off main for its release branch.

This is what has been chosen.