Resolve "Apply BFG Object map might fail due to race condition"

What does this MR do?

Related to: #29536 (closed)

This merge request will give a default value to the lease key used in the garbage collector worker. The worker at the moment may use a nil key, which could collide with another, unrelated service, which uses the same nil key for the ExclusiveLock.

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
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• Style guides
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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