Memoizing root_ancestor in Namespace

What does this MR do?

The root_ancestor method is heavily used in EE for the group_saml feature. Having this memoization implemented in CE would eliminate the need of overriding the root_ancestor method in EE.

Related Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/12189

Related MR in EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14364

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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