Move complex only/except CI/CD configuration syntax to beta or production-ready

Description

Entire section of only/except complex syntax is marked as an alpha feature. See ➡ https://docs.gitlab.com/ee/ci/yaml/#only-and-except-complex

While I believe that we can keep kubernetes: active as an alpha, because we probably want to eventually remove it ➡ https://gitlab.com/gitlab-org/gitlab-ce/issues/52148, variables does not seem alpha. Does it make sense to mark changes as alpha feature too?

Thoughts @ayufan @jlenny?