Install rever from source and clarify bridging branch

[danyeaw]

Description

This PR clarifies some release process steps based on the conda 26.5.3 release:

1. Correct the tag name for conda canary builds (I think we also need to update conda-build to use this format)
2. Ensure rever is installed from source instead of conda-forge
3. Clarify how to make a "bridging branch" for merge conflicts
4. We no longer reopen release issues (so story points can be tracked), so changed it to create a new issue