Add "storage-optimization" to Neptune cluster instance create/update pending states

[jbmchuck]

I've found when a Neptune instance is resized I now get the following
error:

 Error: unexpected state 'storage-optimization', wanted target 'available'. last error: %!s(<nil>)

This is very similar to #15284

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Relates OR Closes #0000

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'

...

[bflad]

Looks good to me, thank you, @jbmchuck 🚀

Output from acceptance testing:

--- PASS: TestAccAWSNeptuneClusterInstance_generatedName (568.53s)
--- PASS: TestAccAWSNeptuneClusterInstance_namePrefix (588.57s)
--- PASS: TestAccAWSNeptuneClusterInstance_withSubnetGroup (754.94s)
--- PASS: TestAccAWSNeptuneClusterInstance_kmsKey (775.82s)
--- PASS: TestAccAWSNeptuneClusterInstance_withaz (834.37s)
--- PASS: TestAccAWSNeptuneClusterInstance_basic (859.18s)

[ghost]

This has been released in version 3.31.0 of the Terraform AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template for triage. Thanks!

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!