Change staging branch name pattern to work better with runbot

[xmo-odoo]

Currently the mergebot has a weird bespoke pattern (staging.{base}) which the runbot does not support out of the box, and needs hacks for in order to detect a trailing post-. version. The mergebot should support more bespoke branch patterns such that it can create branches called {base}-staging or {base}/staging or whatever instead.

Notes:

• this will require branch protection editions, as currently the branch protection is on staging.*
• pattern is TBD and to check when trying to fix this