Disable non-containerized deployments for head

[cbbayburt]

Prevents deploying server and proxy modules with versions head and 5.0-X. Updates the advanced readme to reflect this constraint.

Unfortunately with terraform-1.0.10 it's not possible to print out a specific clear message for this case. Below is an example output when attempting to deploy server on head (same error when using a non-existent product_version):

Error: Invalid index

  on modules/server/main.tf line 40, in module "server":
  40:   image = var.image == "default" ? var.images[var.product_version] : var.image
    ├────────────────
    │ var.images is object with 14 attributes
    │ var.product_version is "head"

The given key does not identify an element in this collection value.

There are some features that help with this in 1.5.x and later. We can revisit this when we finally update to a newer version.

Fixes https://github.com/SUSE/spacewalk/issues/24332