Advertise support for Matrix 1.4. (#14184)

All features / changes in Matrix 1.4 are now supported in
Synapse.

changelog.d/14032.feature

@@ -1 +1 @@
-Advertise Matrix 1.3 support on `/_matrix/client/versions`.
+Advertise support for Matrix 1.3 and 1.4 on `/_matrix/client/versions`.

changelog.d/14184.feature

@@ -0,0 +1 @@
+Advertise support for Matrix 1.3 and 1.4 on `/_matrix/client/versions`.

synapse/rest/client/versions.py

@@ -76,6 +76,7 @@ class VersionsRestServlet(RestServlet):
                     "v1.1",
                     "v1.2",
                     "v1.3",
+                    "v1.4",
                 ],
                 # as per MSC1497:
                 "unstable_features": {