Empty versatile template for CI - ci_syntax_templates_b

What does this MR do?

This replaces the unsuccessful ci_syntax_templates experiment with the ci_syntax_templates_b experiment. The only difference is that the new experiment is targeted to namespaces that are at most 90 days old.

This can be tested locally by visiting a local project and appending the following to the URL:

/-/new/master?commit_message=Add+.gitlab-ci.yml&file_name=.gitlab-ci.yml&force_experiment=ci_syntax_templates_b

For example:

http://localhost:3000/test-user/test-project/-/new/master?commit_message=Add+.gitlab-ci.yml&file_name=.gitlab-ci.yml&force_experiment=ci_syntax_templates_b

Note: there is no changelog, since this is behind a feature flag.

Related to #300993 (closed)