Fixing the rabbitmq_auth_backend_oauth2 schema

[brunomedeirosdedalus]

Fixing reference to the old key 'additional_rabbitmq_scopes'. Removing redundant mapping

Proposed Changes

I am unable to make use of the key 'additional_scopes_key'; It seems to me that is a bug caused by the redundant mapping.

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[michaelklishin]

The second mapping introduced in #6015, possibly as a merge/rebase artifact.

[michaelklishin]

@MarcialRosales FYI. We need to backport this to v3.10.x when it is time to backport #6015 to that branch.