Swap RepositoryHashCache to UNLINK

What does this MR do?

This MR does two things:

1. Issues a single delete call for all keys (which is a bit moot at this point as I believe there's only a single hash key per repo right now, but this is now the same as for other cache adapters and therefore better to standardise).
2. Issues UNLINK calls instead of DEL, as suggested in https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/11018#note_393253950

We no longer support Redis 3 as of 13.0 and so this does not need to be behind a feature flag. Likewise, this has been tested long-term already on Gitlab::SetCache.

Related https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/11018

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