removes experimental warning from praefect settings page

This removes the experimental warning at the top of the praefect settings page as this is production ready, as of version 13.2.x.

Related issues

This was brought to my attention in https://gitlab.zendesk.com/agent/tickets/166717

https://docs.gitlab.com/13.2/omnibus/settings/praefect.html

Changing the header title?

• Is it completely necessary to change the title? Changing titles breaks deep linking. If yes, proceed further.
• Move the old heading title to the bottom of the page.
• Change the header tag to h5, #####.

Moving docs to a new location?

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.