Made google_document_ai_processor_default_version test ignore changes to version field

[melinath]

Resolved hashicorp/terraform-provider-google#12905 by altering the test to ignore changes (and improving documentation.)

projectNumberDiffSuppress is already in place to handle diffs based solely on the project id -> project number change; the larger issue is that the API replaces release channels (now called "stable" and "rc") with specific API versions. Unfortunately, this API does not supply a way to extract the list of versions in a particular release channel, so there's no way to make a data source to get the "latest stable version" and have the resource use that. Fully suppressing the diff in the provider would mislead users into thinking that they are on the latest stable or rc release, when in fact they might be extremely out of date.

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).
• 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.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (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)

[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 ( 3 files changed, 20 insertions(+), 4 deletions(-))
Terraform Beta: Diff ( 3 files changed, 20 insertions(+), 4 deletions(-))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))
TF OiCS: Diff ( 1 file changed, 8 insertions(+), 1 deletion(-))

[modular-magician]

Tests analytics

Total tests: 2467
Passed tests 2209
Skipped tests: 256
Failed tests: 2

Action taken

Triggering VCR tests in RECORDING mode for the tests that failed during VCR. Click here to see the failed tests

TestAccDocumentAIProcessorDefaultVersion_documentaiDefaultVersionExample|TestAccFirebaserulesRelease_BasicRelease

[modular-magician]

Tests passed during RECORDING mode:
TestAccDocumentAIProcessorDefaultVersion_documentaiDefaultVersionExample[Debug log]
TestAccFirebaserulesRelease_BasicRelease[Debug log]

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

[roaks3]

LGTM