POC: Repository Verification refactoring

What does this MR do?

It's a POC of the refactoring where we handle the verification of each repository type separately. The idea here is to allow adding a new type of repository simpler. I also introduced projects.last_wiki_updated_at field to make it more consistent as for now when we push something to wiki we update projects.last_reopository_updated_at which is quite confusing. If we add design repository verification it will be even more confusing.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• 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

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