Ignore dns_config changes for Autopilot clusters

[modular-magician]

Recently, GKE Autopilot has changed the default dns_config to be:

dns_config {
    cluster_dns        = "CLOUD_DNS"
    cluster_dns_domain = "cluster.local"
    cluster_dns_scope  = "CLUSTER_SCOPE"
  }

Customers are not allowed to modify dns_confg in GKE Autopilot. It's a pre-configured feature/config.

But this change still affects Autopilot customers as the terraform tries to converge back to dns_config=null (original default value).

To fix this and to be aligned with the fact that it's not allowed to modify dns_config in Autopilot, this PR makes google_container_cluster to ignore dns_config changes.

Issues:
Fixes hashicorp/terraform-provider-google#15484
Fixes hashicorp/terraform-provider-google#15454

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read Write release notes before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

container: updated `resource_container_cluster` to ignore `dns_config` diff when `enable_autopilot = true` 

Derived from GoogleCloudPlatform/magic-modules#8654

[Demonian]

@modular-magician Thanks for the commit, are there any ETA for provider release with this fix?