change ignore_read on deployment to custom_flatten

[megan07]

Fixes hashicorp/terraform-provider-google#9851

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• 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).
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Release Note Template for Downstream PRs (will be copied)

appengine: fixed bug where `deployment.container.image` would update to an old version even if in `ignore_changes`

[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, 62 insertions(+), 3 deletions(-))
Terraform Beta: Diff ( 4 files changed, 63 insertions(+), 4 deletions(-))

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccComputeRegionBackendService_regionBackendServiceCacheExample|TestAccComputeRegionBackendService_regionBackendServiceExternalExample|TestAccComputeRoute_routeIlbVipExample|TestAccComputeServiceAttachment_serviceAttachmentBasicExampleUpdate|TestAccContainerCluster_withDNSConfig You can view the result here: https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=205281