Partially revert (preferredDomain -> preferred_domain) renaming

Partially reverts 0a6b934a44.

`preferred_domain` is only used by Element and does not apply to the
`/.well-known/matrix/client` file.

roles/custom/matrix-base/templates/static-files/well-known/matrix-client.j2

@@ -20,10 +20,10 @@
 	{% endif %}
 	{% if matrix_client_element_jitsi_preferredDomain %},
 	"io.element.jitsi": {
-		"preferred_domain": {{ matrix_client_element_jitsi_preferredDomain|to_json }}
+		"preferredDomain": {{ matrix_client_element_jitsi_preferredDomain|to_json }}
 	},
 	"im.vector.riot.jitsi": {
-		"preferred_domain": {{ matrix_client_element_jitsi_preferredDomain|to_json }}
+		"preferredDomain": {{ matrix_client_element_jitsi_preferredDomain|to_json }}
 	}
 	{% endif %}
 	{% if matrix_sliding_sync_enabled %},