Configure Cloud Run service to use min TLS 1.2 by default

We should create a default ssl policy with the MODERN profile and TLS set to min 1.2 and attach it to the cloud run service.

Example SSL policy is as follows.

resource "google_compute_ssl_policy" "webapp" {
  name            = "webapp"
  profile         = "MODERN"
  min_tls_version = "TLS_1_2"
}