Remove "KERBEROS" from list of accepted values for "optional_components"

[jpellman]

If you try to add Kerberos to a Dataproc resource (google_dataproc_cluster) using the google-beta provider by including it in the optional_components list, you will receive the following error:

Error: Error creating Dataproc cluster: googleapi: Error 400: Enabling Kerberos via Optional Components and properties is no longer supported. Please configure Kerberos via KerberosConfig instead, badRequest

This patch reflects the information conveyed in this error by removing KERBEROS from the list of potential items that can be included in optional_components.

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[modular-magician]

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[modular-magician]

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

Diff report:

Terraform GA: Diff ( 1 file changed, 1 deletion(-))
Terraform Beta: Diff ( 1 file changed, 1 deletion(-))