Move enable_shielded_nodes from beta to GA.

[nat-henderson]

Fixes hashicorp/terraform-provider-google#6269.

Release Note Template for Downstream PRs (will be copied)

container: Moved `google_container_cluster.enable_shielded_nodes` from beta to GA.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 3 files changed, 66 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 2 files changed, 31 insertions(+), 31 deletions(-))

[slevenick]

Looks like we also need to move the config testAccContainerCluster_withShieldedNodes to GA

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 3 files changed, 78 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 2 files changed, 44 insertions(+), 43 deletions(-))