keycloak: Bump buffer size to make room for response headers

Keycloak is apparently sending over 4k worth of response headers under
some circumstances (maybe when the client sends a stale cookie?), which
causes Nginx to return a 502 error and log "upstream sent too big header
while reading response header from upstream". This is likely also
related to this upstream issue[1]. So bump the buffer to 8k.

[1] https://github.com/keycloak/keycloak/issues/16181

roles/keycloak/templates/nginx.d.conf.j2

@@ -56,6 +56,7 @@ server {
         proxy_set_header    X-Forwarded-For    $remote_addr;
         proxy_set_header    X-Forwarded-Proto  $scheme;
         proxy_pass http://127.0.0.1:{{ keycloak_port }};
+        proxy_buffer_size 8k;
     }
 
     location = / {