Handle bad enum values returned from the GKE API for stack_type

[rileykarson]

Fixes hashicorp/terraform-provider-google#14203

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• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
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• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

container: fixed a bug creating a diff adding a `stack_type` when GKE omitted `stackType` in API responses from older GKE clusters

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 1 file changed, 7 insertions(+))
Terraform Beta: Diff ( 1 file changed, 7 insertions(+))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2552
Passed tests 2273
Skipped tests: 275
Affected tests: 4

Action taken

Found 4 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

TestAccFirebaserulesRelease_BasicRelease|TestAccDataSourceGoogleFirebaseAndroidAppConfig|TestAccApigeeKeystoresAliasesKeyCertFile_apigeeKeystoresAliasesKeyCertFileTestExample|TestAccApigeeEnvKeystoreAliasPkcs12_apigeeEnvKeystoreAliasPkcs12Example

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[modular-magician]

Tests passed during RECORDING mode:
TestAccFirebaserulesRelease_BasicRelease[Debug log]
TestAccDataSourceGoogleFirebaseAndroidAppConfig[Debug log]
TestAccApigeeKeystoresAliasesKeyCertFile_apigeeKeystoresAliasesKeyCertFileTestExample[Debug log]
TestAccApigeeEnvKeystoreAliasPkcs12_apigeeEnvKeystoreAliasPkcs12Example[Debug log]

All tests passed
View the build log or the debug log for each test