By @Hathoute: consider additional_scopes_key when multiple OAuth 2 backends are used (backport #12752)

[mergify]

This is #12751 by @Hathoute. I'm re-submitting it so that all Actions can run with access to GH secrets.

Closes #12750.

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This is an automatic backport of pull request #12752 done by Mergify.

[mergify]

Cherry-pick of 0d51ee9 has failed:

On branch mergify/bp/v4.0.x/pr-12752
Your branch is up to date with 'origin/v4.0.x'.

You are currently cherry-picking commit 0d51ee9ec0.
  (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)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   deps/rabbitmq_auth_backend_oauth2/src/rabbit_oauth2_schema.erl
	both modified:   deps/rabbitmq_auth_backend_oauth2/test/rabbit_oauth2_schema_SUITE.erl

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of f1ee5b5 has failed:

On branch mergify/bp/v4.0.x/pr-12752
Your branch is ahead of 'origin/v4.0.x' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit f1ee5b551a.
  (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)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   deps/rabbitmq_auth_backend_oauth2/src/rabbit_oauth2_schema.erl

no changes added to commit (use "git add" and/or "git commit -a")

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