proxmox-mail-gateway: take the repo suite from the container (#18475)

* 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.

* fix(proxmox-mail-gateway): read the codename inline

install/proxmox-mail-gateway-install.sh

@@ -16,9 +16,9 @@ update_os
 msg_info "Installing Proxmox Mail Gateway"
 setup_deb822_repo \
   "pmg" \
-  "https://enterprise.proxmox.com/debian/proxmox-release-trixie.gpg" \
+  "https://enterprise.proxmox.com/debian/proxmox-release-$(get_os_info codename).gpg" \
   "http://download.proxmox.com/debian/pmg" \
-  "trixie" \
+  "$(get_os_info codename)" \
   "pmg-no-subscription"
 $STD apt install -y proxmox-mailgateway-container
 msg_ok "Installed Proxmox Mail Gateway"