Coming from .github#13, standardize release branching to match what OpenSearch is doing.

1. Create a 1.0 branch for the OpenSearch 1.0 release, 1.x branch for next 1.1 release, and use main for 2.0 development. Make sure CI is enabled on those.
2. Update your release/branching documentation. If you don't have one, add a RELEASING.md that links to, or has content from .github/RELEASING.md. I did this in Cleanup READMEs. #32
3. Delete any merged/stale/develop/feature branches that are no longer in use.
4. Communicate any changes in process to your team.

Refer to release branching for more information.