rabbitmq_management: Fix issue with oauth2+basic_auth (backport #15793) (backport #15832)

[mergify]

Proposed Changes

Fix: OAuth2 + basic auth — basic-auth users lost stored credentials on reload

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• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

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• Mandatory: I (or my employer/client) have have signed the CA (see https://github.com/rabbitmq/cla)
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• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
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• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

Further Comments

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This is an automatic backport of pull request #15793 done by [Mergify](https://mergify.com).

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This is an automatic backport of pull request #15832 done by [Mergify](https://mergify.com).