WPCOM MU: Add admin notice to know about hosting menu (#35930)

* WPCOM MU: Add dismissible admin banner for site menu

* changelog

* WPCOM MU: Add helper function for displaying the notice and only show on dashboard

* Hide while loading

* Check for flag enabled & typo fix

---------

Co-authored-by: Gabriel Demichelis <[email protected]>

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/8043018299

Author: ebinnion