Fix: Invalidate retries with the invalidation of an Outbox Item

[pfefferle]

This PR also invalidates all retries for an invalidated Outbox-Item.

This is nearly impossible to test, because phpunit does not let me create schedules.

Proposed changes:

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Go to '..'

Changelog entry

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Changelog Entry Details

Significance

• Patch
• Minor
• Major

Type

• Added - for new features
• Changed - for changes in existing functionality
• Deprecated - for soon-to-be removed features
• Removed - for now removed features
• Fixed - for any bug fixes
• Security - in case of vulnerabilities

Message

Properly remove retries schedules, with the invalidation of an Outbox-Item.

[obenland]

@pfefferle I understand you technically can't approve the changes I made, but it would be good if you could still review and test them on your end.

I updated it to use the new hook name, consolidate schedule management in the Scheduler, and add a unit test to make sure it works.