rabbit_backing_queue: pass mc:state() to discard callback (backport #13382)

[mergify]

Proposed Changes

As requested in #13374, passing mc:state() to rabbit_backing_queue:discard() callback.

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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)
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This is an automatic backport of pull request #13382 done by [Mergify](https://mergify.com).