Re-enable ci_support_interruptible_pipelines feature flag

There was some confusion in gitlab-org/gitlab#32022 (comment 220326484) whether GitLab.com supported interruptible pipelines, which shipped in 12.3.0. We had to disable it due to gitlab-org/gitlab#31776 (closed).

We should re-enable this now that the issue has been solved.

/cc: @ayufan, @jlenny