Revert "nginx update v1.21.2"

This reverts commit 732051b8fc.

There's no such container image published yet.

roles/matrix-nginx-proxy/defaults/main.yml

@@ -1,5 +1,5 @@
 matrix_nginx_proxy_enabled: true
-matrix_nginx_proxy_version: 1.21.2-alpine
+matrix_nginx_proxy_version: 1.21.1-alpine
 
 # We use an official nginx image, which we fix-up to run unprivileged.
 # An alternative would be an `nginxinc/nginx-unprivileged` image, but