Swap alpha for experimental and introduce additional guard rails

We have switched from Alpha to Experiment, see https://docs.gitlab.com/ee/policy/alpha-beta-support.html. This MR tries to align this, although there are other changes required to other pages like:

• https://about.gitlab.com/support/statement-of-support/#alpha--beta-features
• https:/docs.gitlab.com/ee/policy/alpha-beta-support.html#support-for-alpha-beta-and-generally-available-features