Change the import behaviour of GCE instance metadata startup-script

[rileykarson]

Fixes hashicorp/terraform-provider-google#4388

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Release Note Template for Downstream PRs (will be copied)

compute: changed the import / drift detection behaviours for `metadata_startup_script`, `metadata.startup-script` in `google_compute_instance`. Now, `metadata.startup-script` will be set by default, and `metadata_startup_script` will only be set if present.

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 5 files changed, 41 insertions(+), 21 deletions(-))
Terraform Beta: Diff ( 5 files changed, 41 insertions(+), 21 deletions(-))

[rileykarson]

Note: no related failures in https://ci-oss.hashicorp.engineering/buildConfiguration/GoogleCloudBeta_ProviderGoogleCloudBetaMmUpstreamVcr/210882?

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 5 files changed, 41 insertions(+), 21 deletions(-))
Terraform Beta: Diff ( 6 files changed, 42 insertions(+), 22 deletions(-))