Standardize branching to match OpenSearch

[dblock]

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.
3. If you are using a develop branch, stop. You should be using the default main brach for furthermost, future development.
4. Delete any merged/stale/develop/feature branches that are no longer in use.
5. Communicate any changes in process to your team.

Refer to release branching for more information.