Update default room version to 6 (#8461)

Per https://github.com/matrix-org/matrix-doc/pull/2788

changelog.d/8461.feature

@@ -0,0 +1 @@
+Change default room version to "6", per [MSC2788](https://github.com/matrix-org/matrix-doc/pull/2788).

docs/sample_config.yaml

@@ -119,7 +119,7 @@ pid_file: DATADIR/homeserver.pid
 # For example, for room version 1, default_room_version should be set
 # to "1".
 #
-#default_room_version: "5"
+#default_room_version: "6"
 
 # The GC threshold parameters to pass to `gc.set_threshold`, if defined
 #

synapse/config/server.py

@@ -39,7 +39,7 @@ logger = logging.Logger(__name__)
 # in the list.
 DEFAULT_BIND_ADDRESSES = ["::", "0.0.0.0"]
 
-DEFAULT_ROOM_VERSION = "5"
+DEFAULT_ROOM_VERSION = "6"
 
 ROOM_COMPLEXITY_TOO_GREAT = (
     "Your homeserver is unable to join rooms this large or complex. "

tests/rest/client/v1/test_directory.py

@@ -21,6 +21,7 @@ from synapse.types import RoomAlias
 from synapse.util.stringutils import random_string
 
 from tests import unittest
+from tests.unittest import override_config
 
 
 class DirectoryTestCase(unittest.HomeserverTestCase):
@@ -67,10 +68,18 @@ class DirectoryTestCase(unittest.HomeserverTestCase):
         self.ensure_user_joined_room()
         self.set_alias_via_directory(400, alias_length=256)
 
-    def test_state_event_in_room(self):
+    @override_config({"default_room_version": 5})
+    def test_state_event_user_in_v5_room(self):
+        """Test that a regular user can add alias events before room v6"""
         self.ensure_user_joined_room()
         self.set_alias_via_state_event(200)
 
+    @override_config({"default_room_version": 6})
+    def test_state_event_v6_room(self):
+        """Test that a regular user can *not* add alias events from room v6"""
+        self.ensure_user_joined_room()
+        self.set_alias_via_state_event(403)
+
     def test_directory_in_room(self):
         self.ensure_user_joined_room()
         self.set_alias_via_directory(200)