make credit_types_treatment in google_billing_budget resource updatable

[modular-magician]

This fixes hashicorp/terraform-provider-google#12886
Internal ticket: b/253294638

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Release Note Template for Downstream PRs (will be copied)

billingbudget: fixed a bug where `budget_filter.credit_types_treatment` in `google_billing_budget` resource was not updating.

Derived from GoogleCloudPlatform/magic-modules#6763