💅 Add a config for the Patchback GitHub App (#12475)
This patch prepares the project's backporting process to start being
handled by the Patchback GitHub App [[1]].
Ref #9384
Resolves #9385
Resolves #9553
Resolves #9554
Resolves #9555
[1]: https://github.com/apps/patchback
(cherry picked from commit d7b401063874141fcbcda78eaec3c28148e7dcd8)