Disable automerge on private repositories

It turns out that GitHub doesn't support auto-merging pull requests on
private repositories [1]. The request to set the attribute is accepted
without error, but doesn't actually change anything. This causes noise in
the output of `tofu apply` since it's constantly trying to re-apply the
change.

Update the `common_repository` module so that we set
`allow_auto_merge=false` for private repositories.

[1]: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/incorporating-changes-from-a-pull-request/automatically-merging-a-pull-request

modules/common_repository/main.tf

@@ -3,7 +3,7 @@ resource "github_repository" "repo" {
   visibility       = var.visibility
   description      = var.description
   auto_init        = true
-  allow_auto_merge = true
+  allow_auto_merge = var.visibility == "private" ? false : true
   has_issues       = true
   has_downloads    = false
   has_projects     = false