Clean up Experimental usage and come up with a graduation policy

Beam, APIs use experimental features quite a lot. Experimental features are growing, but at some point we should consider either removing or graduating them.

This is a proposal to:

• Track: All experimental features will be continually tracked in the change log explicitly.
• For features staying in experimental mode longer than 12 months,
  --> Graduate: we should review whether the feature meets the usage bar to be long term maintained,
  • > Deprecate: or should be deprecated to reduce maintenance load.

It would be good to add a github ci action to check new usage of experimental tags are added to the change log.

/cc kenn lcwik robertwb