fix pubsub schema update failure

[shuyama1]

Pub/Sub Schemas resource does not support updating after creation and it must be destroyed and recreated while currently, it is not handled correctly.
Fixes hashicorp/terraform-provider-google#10718

If this PR is for Terraform, I acknowledge that I have:

• 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).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• 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).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

pubsub: changed `google_pubsub_schema` so that modifiying fields will recreate the resource rather than causing Terraform to report it would attempt an invalid update

pubsub: fix update failure when attempting to change non-updatable resource `google_pubsub_schema`

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 3 files changed, 20 insertions(+), 79 deletions(-))
Terraform Beta: Diff ( 3 files changed, 20 insertions(+), 79 deletions(-))

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDatasourceGoogleServiceNetworkingPeeredDnsDomain_basic|TestAccApigeeEnvironmentIamBindingGenerated|TestAccApigeeEnvironmentIamPolicyGenerated|TestAccCloudFunctionsFunction_vpcConnector|TestAccContainerNodePool_withInvalidUpgradeSettings|TestAccServiceNetworkingPeeredDNSDomain_basic You can view the result here: https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=231803

[rileykarson]

LGTM- I added an extra changelog note, just to be extra cautious (this is technically a breaking change, but since it always resulted in an error we should be fine to do it in a minor)