Group project templates should be a Silver/Premium feature - CE backport

What does this MR do?

• Correct the bug and make group-level project templates only available for Premium/Silver.
• Allow any Free groups currently using the feature to continue using it for a limited time (3 months). At the end of the 3 months, we enforce this for all groups. https://gitlab.com/gitlab-org/gitlab-ee/issues/10926

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

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