[8.x] Adds 8.16 branch to backport and mergify configuration (backport #2861)

[mergify]

Overview

This PR updates configuration files to enable automatic backports to the 8.16 branch.

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This is an automatic backport of pull request #2861 done by Mergify.

[mergify]

Cherry-pick of 7978a9a has failed:

On branch mergify/bp/8.x/pr-2861
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit 7978a9a9.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   .mergify.yml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .backportrc.json

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally