VPC: fix explicit DependsOn: GatewayAttachment
VPC: fix explicit DependsOn: GatewayAttachment

`DependsOn` is only required for implicit dependencies that aren't
represented by things like `Ref: Foo`. In this case, I believe it's just
the GatewayAttachment (associating the InternetGateway with the VPC)
which needs to be specified.

holmesjr/elastic-ci-stack-for-aws,buildkite/elastic-ci-stack-for-aws,holmesjr/elastic-ci-stack-for-aws