provider/google: replace instance group manager urls with instance group urls in container cluster tests

[danawillow]

Follow-up to #12486. Fixes a bunch of currently-failing tests because of the interaction between #12486 and #12147.

[danawillow]

TF_ACC=1 go test ./builtin/providers/google -v -run=TestAccContainerCluster -timeout 120m
=== RUN   TestAccContainerCluster_basic
--- PASS: TestAccContainerCluster_basic (334.04s)
=== RUN   TestAccContainerCluster_withAdditionalZones
--- PASS: TestAccContainerCluster_withAdditionalZones (385.93s)
=== RUN   TestAccContainerCluster_withVersion
--- FAIL: TestAccContainerCluster_withVersion (0.40s)
	testing.go:273: Step 0 error: Error applying: 1 error(s) occurred:

		* google_container_cluster.with_version: 1 error(s) occurred:

		* google_container_cluster.with_version: googleapi: Error 400: Version is invalid., badRequest
=== RUN   TestAccContainerCluster_withNodeConfig
--- PASS: TestAccContainerCluster_withNodeConfig (343.54s)
=== RUN   TestAccContainerCluster_withNodeConfigScopeAlias
--- PASS: TestAccContainerCluster_withNodeConfigScopeAlias (323.48s)
=== RUN   TestAccContainerCluster_network
--- PASS: TestAccContainerCluster_network (385.83s)
=== RUN   TestAccContainerCluster_backend
--- PASS: TestAccContainerCluster_backend (316.05s)

(the version one will clearly have a different fix)

[paddycarver]

LGTM. Merge at will. Thank you for greening tests, you're the best. 😁

[ghost]

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