Update reduce repo size documentation

What does this MR do?

Updates and clarifies our Repository Size Reduction documentation.

"Purge files from repository history" can be confusing because:

• these steps do not reduce repository size on disk or in the UI/API/DB
• this process is not required for Repository Cleanup to work, and may prevent Repository Cleanup from working as expected
• steps 4-6 fail with fatal: --mirror can't be combined with refspecs

This MR consolidates the two sets of documentation into one process that rewrites git history and reduces storage.

@nick.thomas

See: https://gitlab.com/gitlab-org/gitlab/-/issues/224736#note_495409760 Closes https://gitlab.com/gitlab-org/gitlab/-/issues/221326

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