fix(proxmox-mail-gateway): take the repo suite from the container

The install hardcoded trixie, so a Debian 12 container got the PMG 9
repository and pmg-api could not resolve its trixie dependencies (held
broken packages). Suite and key now follow the container's codename;
bookworm still has PMG 8 with proxmox-mailgateway-container.

install/proxmox-mail-gateway-install.sh

@@ -14,11 +14,12 @@ network_check
 update_os
 
 msg_info "Installing Proxmox Mail Gateway"
+CODENAME="$(get_os_info codename)"
 setup_deb822_repo \
   "pmg" \
-  "https://enterprise.proxmox.com/debian/proxmox-release-trixie.gpg" \
+  "https://enterprise.proxmox.com/debian/proxmox-release-${CODENAME}.gpg" \
   "http://download.proxmox.com/debian/pmg" \
-  "trixie" \
+  "${CODENAME}" \
   "pmg-no-subscription"
 $STD apt install -y proxmox-mailgateway-container
 msg_ok "Installed Proxmox Mail Gateway"