Empty pattern field disabled cleanup work <13.6

What does this MR do?

In older versions of GitLab, the form for adding cleanup policies showed a .* text-box placeholder text for the necessary field Remove tags matching that mislead users into thinking it was a default value. When submitted, the placeholder did not default to its shown value and caused a nil value to be stored.

A nil value for the remove tags pattern effectively meant no tags would get removed when the policy ran.

This has been fixed in 13.6 (placeholder also acts as default) and 13.7 (revamp of UI).

Change adds a note field to the end of the UI doc section.

Screenshots (strongly suggested)

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