Make missing /_synapse/admin correctly return 404 responses

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1058

We may try to capture such calls and return a friendlier response (HTML
or JSON) saying "The Synapse Admin API is not enabled", but that may not
be desirable.

For now, we stick to what "upstream" recommends: "simply
don't proxy these APIs", which should lead to the same kind of 404 that
we have now.
See here: 6660912226/docs/reverse_proxy.md (synapse-administration-endpoints)

roles/matrix-nginx-proxy/templates/nginx/conf.d/matrix-domain.conf.j2

@@ -136,7 +136,13 @@
 		proxy_max_temp_file_size 0;
 	}
 
-	location / {
+	{#
+		We only handle the root URI for this redirect or homepage serving.
+		Unhandled URIs (mostly by `matrix_nginx_proxy_proxy_matrix_client_api_forwarded_location_prefix_regexes` above) should result in a 404,
+		instead of causing a redirect.
+		See: https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1058
+	#}
+	location ~* ^/$ {
 		{% if matrix_nginx_proxy_proxy_matrix_client_redirect_root_uri_to_domain %}
 			return 302 $scheme://{{ matrix_nginx_proxy_proxy_matrix_client_redirect_root_uri_to_domain }}$request_uri;
 		{% else %}