fix: use backend.key (not each.key)

What

Use backend.key instead of each.key.

Why

Using each.key leads to the following error:

│ Error: Reference to "each" in context without for_each
│ 
│   on modules/internal-loadbalancer/main.tf line 58, in resource "google_compute_backend_service" "default":
│   58:       group = each.key
│ 
│ The "each" object can be used only in "module" or "resource" blocks, and
│ only when the "for_each" argument is set.