[Fix] Fix creation of databricks_storage_credential and databricks_credential resources on GCP with isolation mode (#4563)

## Changes
<!-- Summary of your changes that are easy to understand -->

Resolves #4562 

## Tests
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How is this tested? Please see the checklist below and also describe any
other relevant tests
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- [x] `make test` run locally
- [ ] relevant change in `docs/` folder
- [ ] covered with integration tests in `internal/acceptance`
- [ ] using Go SDK
- [ ] using TF Plugin Framework

Co-authored-by: vuong-nguyen <[email protected]>

Author: alexott

NEXT_CHANGELOG.md

@@ -6,6 +6,7 @@
 
 ### Bug Fixes
 
+ * Fix creation of `databricks_storage_credential` and `databricks_credential` resources on GCP with isolation mode ([#4563](https://github.com/databricks/terraform-provider-databricks/pull/4563))
  * Handle auto-enabled errors with `databricks_system_schema` [#4547](https://github.com/databricks/terraform-provider-databricks/pull/4547)
  * Skip Read after Create in `databricks_secret_acl` to avoid errors([#4548](https://github.com/databricks/terraform-provider-databricks/pull/4548)).
 

catalog/resource_credential.go

@@ -82,6 +82,9 @@ func ResourceCredential() common.Resource {
 			var update catalog.UpdateCredentialRequest
 			common.DataToStructPointer(d, credentialSchema, &update)
 			update.NameArg = d.Id()
+			if update.DatabricksGcpServiceAccount != nil { // we can't update it at all
+				update.DatabricksGcpServiceAccount = nil
+			}
 			_, err = w.Credentials.UpdateCredential(ctx, update)
 			if err != nil {
 				return err

catalog/resource_storage_credential.go

@@ -48,6 +48,9 @@ func ResourceStorageCredential() common.Resource {
 			common.DataToStructPointer(d, storageCredentialSchema, &create)
 			common.DataToStructPointer(d, storageCredentialSchema, &update)
 			update.Name = d.Get("name").(string)
+			if update.DatabricksGcpServiceAccount != nil { // we can't update it at all
+				update.DatabricksGcpServiceAccount = nil
+			}
 
 			return c.AccountOrWorkspaceRequest(func(acc *databricks.AccountClient) error {
 				storageCredential, err := acc.StorageCredentials.Create(ctx,