Restore self-building on ARM for Synapse for now

While v1.22.0 supposedly has multi-arch Docker images
(thanks to matrix-org/synapse#7921),
I can't them on Docker Hub yet, so I'm backing out of this change
for now and letting people fall back to self-building there.

group_vars/matrix_servers

@@ -917,7 +917,7 @@ matrix_client_element_jitsi_preferredDomain: "{{ matrix_server_fqn_jitsi if matr
 #
 ######################################################################
 
-matrix_synapse_container_image_self_build: "{{ matrix_architecture not in ['arm32', 'arm64', 'amd64'] }}"
+matrix_synapse_container_image_self_build: "{{ matrix_architecture not in ['amd64'] }}"
 
 # When ma1sd is enabled, we can use it to validate email addresses and phone numbers.
 # Synapse can validate email addresses by itself as well, but it's probably not what we want by default when we have an identity server.