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>

install/emqx-install.sh

@@ -38,6 +38,18 @@ rm -f "$DEB_FILE"
 echo "$LATEST_VERSION" >~/.emqx
 msg_ok "Installed EMQX"
 
+read -r -p "${TAB3}Would you like to disable the EMQX MQ feature? (reduces disk/CPU usage) <y/N> " prompt
+if [[ ${prompt,,} =~ ^(y|yes)$ ]]; then
+  msg_info "Disabling EMQX MQ feature"
+  mkdir -p /etc/emqx
+  if ! grep -q "^mq.enable" /etc/emqx/emqx.conf 2>/dev/null; then
+    echo "mq.enable = false" >>/etc/emqx/emqx.conf
+  else
+    sed -i 's/^mq.enable.*/mq.enable = false/' /etc/emqx/emqx.conf
+  fi
+  msg_ok "Disabled EMQX MQ feature"
+fi
+
 msg_info "Starting EMQX service"
 $STD systemctl enable -q --now emqx
 msg_ok "Enabled EMQX service"