Advertise supporting version 1.3 of the Matrix spec. (#14032)

Now that all features / changes in 1.3 are supported in Synapse.

changelog.d/14032.feature

+Advertise Matrix 1.3 support on `/_matrix/client/versions`.

synapse/rest/client/versions.py

@@ -75,6 +75,7 @@ class VersionsRestServlet(RestServlet):
                     "r0.6.1",
                     "v1.1",
                     "v1.2",
+                    "v1.3",
                 ],
                 # as per MSC1497:
                 "unstable_features": {