Google has removed the Google Hangouts brand, thus we are removing
them as video chat provider option.
This commit removes Google Hangouts integration and make a migration
that sets all realms that are using Hangouts as their video chat
provider to the default, jitsi.
With changes by tabbott to improve the overall video call documentation.
Fixes: #15298.
Zixuan James Li
Anders Kaseorg
Clara Dantas