💅 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

Author: webknjaz

.github/patchback.yml

@@ -0,0 +1,7 @@
+---
+
+backport_branch_prefix: patchback/backports/
+backport_label_prefix: 'backport '  # IMPORTANT: the labels are space-delimited
+# target_branch_prefix: ''  # The project's backport branches are non-prefixed
+
+...