service is part of the URL and therefore is required.

[nat-henderson]

Release Note Template for Downstream PRs (will be copied)

`app_engine_flexible_app_version` and `app_engine_standard_app_version` now require `service` to be set.  Before this change, leaving the value empty would have passed validation but failed during apply.

Fixes hashicorp/terraform-provider-google#6711

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 4 files changed, 20 insertions(+), 20 deletions(-))
Terraform Beta: Diff ( 4 files changed, 20 insertions(+), 20 deletions(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=127012"