Swap to UNLINK for Redis set cache

What does this MR do?

This uses UNLINK in Redis 4 to delete sets. This unlinks the key from the value but actually deletes the value asynchronously. With large sets this should give us a reasonable performance boost.

This MR also implements multi-key deletes for RepositorySetCache methods in the Repository model, meaning only one UNLINK call will be issued for all sets being expired.

Implements :repository_set_cache_unlink feature flag, defaulting to on.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team