Partially revert #11602 to prevent webclient overriding client resource

known client resource prefixes under one, simplified '/_matrix/client'
prefix. Unfortunately, '/_matrix/client' is already a prefix used by the
'webclient' resource. As we use a Python dictionary to hold a mapping
from prefix to resource, configuring a 'webclient' resource after a
'client' resource would end up with all client requests being redirected
to the webclient resource (which doesn't know how to handle things like
/login or /sync).

Reverting #11602 fixes the bug while maintaining JSON responses as the
PR intended. However, this bug has highlighted how fragile the resource code
is currently.

synapse/app/homeserver.py

@@ -27,7 +27,6 @@
 import synapse.config.logger
 from synapse import events
 from synapse.api.urls import (
-    CLIENT_API_PREFIX,
     FEDERATION_PREFIX,
     LEGACY_MEDIA_PREFIX,
     MEDIA_R0_PREFIX,
@@ -193,7 +192,13 @@ def _configure_named_resource(
 
             resources.update(
                 {
-                    CLIENT_API_PREFIX: client_resource,
+                    "/_matrix/client/api/v1": client_resource,
+                    "/_matrix/client/r0": client_resource,
+                    "/_matrix/client/v1": client_resource,
+                    "/_matrix/client/v3": client_resource,
+                    "/_matrix/client/unstable": client_resource,
+                    "/_matrix/client/v2_alpha": client_resource,
+                    "/_matrix/client/versions": client_resource,
                     "/.well-known": well_known_resource(self),
                     "/_synapse/admin": AdminRestResource(self),
                     **build_synapse_client_resource_tree(self),