Update Staging Ref URL

What does this MR do?

Staging Ref env moved from staging-ref.testbed.gitlab.net to staging-ref.gitlab.com - it's needed to get a correct result from Specs::Helpers::ContextSelector.dot_com? to enable skip_live_env tag in tests.

Related issues

• https://gitlab.com/gitlab-org/gitlab/-/issues/338977#note_716425383