Fix fatal error on admin settings page: OC\Server magic getters removed

[nd-]

Problem

On Nextcloud core versions where the legacy magic getters have been removed from OC\Server (confirmed on Nextcloud 34.0.1), loading the admin settings page at /settings/admin/richdocumentscode throws a fatal error:

Call to undefined method OC\Server::getURLGenerator()
  in apps/richdocumentscode/lib/Settings/Admin.php:36

Admin::getForm() also calls \OC::$server->getAppManager() (line 45), which is affected the same way and would fail right after the first call is fixed.

info.xml declares max-version="35", so the app is expected to work on this core version, but these two direct magic-getter calls were never migrated to the DI container's get() method like the rest of the app already does elsewhere.

Fix

Replace both calls with \OC::$server->get(SomeInterface::class), which is the supported way to fetch services from the container and remains stable across core versions (confirmed present via SimpleContainer::get()).

Testing

Verified on a Nextcloud 34.0.1 instance: the admin settings page now renders without error after this change.

[nd-]

Duplicate of #345, which fixes the same underlying issue (#344) in the same file. Closing in favor of that PR — sorry for the noise, should have searched for existing PRs/issues first.