Remove username and password from master_auth

[modular-magician]

Fixes hashicorp/terraform-provider-google#8032

Removes basic auth from container cluster. Revives one of the tests removed here: https://github.com/hashicorp/terraform-provider-google-beta/pull/3462/files#diff-bd126349706a4e5427118dc65de0dd5ec0cb74bdf11c6ef5cf967d3a2f3244b6L2662

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Release Note Template for Downstream PRs (will be copied)

container: removed `master_auth.username` and `master_auth.password` from `google_container_cluster`

container: made `master_auth.client_certificate_config` required

Derived from GoogleCloudPlatform/magic-modules#5372