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

[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)
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• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

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Further Comments

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