Reset templates cache key

What does this MR do?

This resets description templates cache key, thus making templates being reloaded and recached with the correct format. The problem appeared when we deployed !52360 (merged) (merged) where we should have changed the cache key as here we are filtering templates by project_id which is not yet present in the returned hash introduced in !52222 (diffs) but there the project_id is always nil

Here is a production snippet run for Gitlab project

repository_cache = Gitlab::RepositoryCache.new(Project.find(278964).repository)
repository_cache.fetch(:merge_request_template_names_by_category)

=>   { 
    "" =>[
      {:name=>"Change Documentation Location", :id=>"Change Documentation Location", :key=>"Change Documentation Location", :project_id=>nil}, 
      {:name=>"Documentation", :id=>"Documentation", :key=>"Documentation", :project_id=>nil}, 
      {:name=>"New End To End Test", :id=>"New End To End Test", :key=>"New End To End Test", :project_id=>nil}, 
      {:name=>"New Static Analysis Check", :id=>"New Static Analysis Check", :key=>"New Static Analysis Check", :project_id=>nil}, 
      {:name=>"Security Release", :id=>"Security Release", :key=>"Security Release", :project_id=>nil}
    ]
  }

You can note the :project_id=>nil in the result, which makes the list of templates to be empty.

#322078 (closed)

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