docs: Update stable process around using fixes: and gitlab

docs: Update stable process around using fixes: and gitlab

Currently the docs still recommend using
mesa-stable@lists.freedesktop.org, which is pretty awful. We really
don't want a second mailing list and it's mostly full of junk because of
CC: tags anyway.
This changes the preferred actions to be:
1) use a fixes: tag ahead of time
2) use a Cc tag ahead of time if fixes isn't appropriate
3) Use a gitlab MR against the staging/ branch for post-merge/backport
   nominations

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3056>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3056>