This adds support for the `secure_backup_setup_methods` key, which allows HS
admins to state that Element should simplify down to only one setup method,
rather than offering both.
Fixes https://github.com/vector-im/element-web/issues/15238
If the Secure Backup required mode is set the client `.well-known` file, then
this will ensure that everyone already inside the app is required to complete
setup matching that policy.
Fixes https://github.com/vector-im/element-web/issues/14954