dendrite: enable MSC2946 (Spaced Summary) by default

roles/matrix-dendrite/templates/dendrite/dendrite.yaml.j2

@@ -293,7 +293,8 @@ mscs:
   # Currently valid values are:
   # - msc2836    (Threading, see https://github.com/matrix-org/matrix-doc/pull/2836)
   # - msc2946    (Spaces Summary, see https://github.com/matrix-org/matrix-doc/pull/2946)
-  mscs: []
+  mscs:
+    - msc2946
   database:
     connection_string: {{ matrix_dendrite_database_str }}/{{ matrix_dendrite_mscs_database }}?sslmode=disable
     max_open_conns: 5