T8218: removed unqueue from mergify yaml as it is not supported

[kumvijaya]

Change summary

Removed unqueue from mergify yaml as it is not supported as it was blocking the backport.
Reference: https://docs.mergify.com/changelog/2023-11-13-rename-unqueue-command-to-dequeue/

Background:
.mergify.yml config file, referring to unqueue which has been renamed to dequeue since November 2023 (https://docs.mergify.com/changelog/2023-11-13-rename-unqueue-command-to-dequeue/).
Mergify recently dropped the outdated unqueue alias, which means config file is now invalid, so mergify engine is not able to process the events.

Remove the command_restrictions.unqueue key from your config file, and things should go back to normal.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

https://vyos.dev/T8218

Related PR(s)

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• I have run the components SMOKETESTS if applicable
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• My change requires a change to the documentation
• I have updated the documentation accordingly

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