Move change_repository_storage policy to BasePolicy

What does this MR do?

As we move towards supporting the moving of a Snippet's repository storage, the access policy for change_repository_storage will no longer only apply to Project, it will also be required for Project and Personal Snippets.

Currently, the policy is defined in the ProjectPolicy, but this MR moves it into the BasePolicy, to enable it to also be available for these other resources.

Refs #218986 (closed)

Additional tests have been added to the policy spec (none were present in the ProjectPolicy spec), but it's worth noting that existing use of this policy is also covered in the Project API specs and Projects UpdateService specs

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.