Switch Release branch policy to always match up 1-to-1 with pulpcore release branches

[gerrod3]

Currently this repo's release branches are always one behind the latest release for tagging purposes, but this is confusing to outside contributors. Switch it to be aligned with pulpcore (and all other plugins) release branching strategy.

[mikedep333]

FYI: This involves the main branch being only used to build nightly, not to build stable.

[decko]

https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#create

This could help