[BUG] members_can_create_internal_repositories in github_organization_settings missing Computed: true causing phantom diffs

When a user omits `members_can_create_internal_repositories` from their config and the enterprise controls the setting externally, every plan shows a diff (state `true` to zero-value `false`). Applying that diff sends an update that fails with 422 because the enterprise policy won't allow the change.

The root cause is the same as #3360: the field is `Optional` without `Computed`, so the Read function writing the API value back into state conflicts with Terraform's zero-value assumption. Adding `Computed: true` fixes it.