Resolve "Deprecate `PersonalAccessTokens ::PolicyWorker` in favor of `PersonalAccessTokens::Instance::PolicyWorker`"

What does this MR do?

For: #213791 (closed)

PersonalAccessTokens::PolicyWorker is no longer used, and is replaced by PersonalAccessTokens::Instance::PolicyWorker from GitLab 12.10 onwards.

However, PersonalAccessTokens::PolicyWorker was kept intact just in case there were old jobs from this worker that were enqueued before 12.10 - so that after 12.10 is deployed and if the job is taken out from the queue for processing, it does not fail and processes the job as intended.

From 12.10 onwards, new jobs are processed by the PersonalAccessTokens::Instance::PolicyWorker, so it is safe to remove the old worker.

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

Closes #213791 (closed)