master_ipv4_cidr_block should be in line with documentation

[modular-magician]

As per https://cloud.google.com/composer/docs/reference/rest/v1beta1/projects.locations.environments#PrivateClusterConfig masterIpv4CidrBlock is Optional. The CIDR block from which IPv4 range for GKE master will be reserved. If left blank, the default value of '172.16.0.0/23' is used. Suggesting change default value to be in line with docs or remove it so it's assigned/computed by API.

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Release Note Template for Downstream PRs (will be copied)

composer: changed master_ipv4_cidr_block to draw default from the API

Derived from GoogleCloudPlatform/magic-modules#4732

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