Fix fetching of app_engine_standard_app_version data

[modular-magician]

Release Note Template for Downstream PRs (will be copied)

appengine: added ability to fully sync `StandardAppVersion` resources

@megan07 I've tried to match the generated terraform output to my original PR #6388

I didn't try to add your suggested

      handlers: !ruby/object:Overrides::Terraform::PropertyOverride
        default_from_api: true

since I don't have any appengine versions with just the default handlers to try with.
Also I don't currently have FlexibleAppVersion resources setup in terraform to play around with.

Maybe this PR can serve as base for more changes.

References

• google_appengine_standard_version is missing properties on read #6417
• Fix fetching of app_engine_standard_app_version data #6388

Derived from GoogleCloudPlatform/magic-modules#3536

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