fix(emqx): increase disk to 6GB and add optional MQ disable prompt (#11844)

EMQX 6.1+ preallocates significant disk space for the MQ feature,
causing high CPU/disk usage on small containers (emqx/emqx#16649).

- Increase default disk from 4GB to 6GB
- Add read -rp prompt during install to optionally disable MQ feature
  via mq.enable=false in emqx.conf (reduces disk/CPU overhead)
- Setting is in install script (not CT script) per reviewer feedback

Co-authored-by: sim-san <sim-san@users.noreply.github.com>

frontend/public/json/emqx.json

@@ -21,7 +21,7 @@
       "resources": {
         "cpu": 2,
         "ram": 1024,
-        "hdd": 4,
+        "hdd": 6,
         "os": "debian",
         "version": "13"
       }