Remove relay_mode field support

[modular-magician]

fixes #17997

relay_mode field from AdvancedDatapathObservabilityConfig is superseded by enable_relay field and will be deprecated on the provider side in near future.

This is a breaking change according to TF docs and as such, should be released in the next major release. Deprecation of relay_mode field on TF side has already happened more than a month ago, foreshadowing this change through deprecation message.

This field is part of a provider feature that is still in Public Preview and yet to go GA.

container: removed deprecated field `advanced_datapath_observability_config.relay_mode` from `google_container_cluster` resource. Users are expected to use `enable_relay` field instead.

container: made field `advanced_datapath_observability_config.enable_relay` required in `google_container_cluster` resource

Derived from GoogleCloudPlatform/magic-modules#10274

[github-actions]

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