Mark User Info Fetcher as not experimental anymore

[lfrancke]

We've shipped the UIF for a couple of releases now and it is in production with various customers. Some of the backends itself can stay experimental for now but the UIF itself seems to be working and stable.

• Update the documentation => chore: Mark user-info-fetcher as non-experimental #752
• Update our Experimental Feature Tracker Github Project -> Remove this issue
• Add a release note

[sbernauer]

Release notes

The User info fetcher is no longer an experimental feature, as it has been used in production for ~1 year without problems