Fix composer environment version regexp

[modular-magician]

Fixed environment version regexp to explicitly require . (dot) instead of any character after preview literal.
Example: composer-2.0.0-preview.0-airflow-2.1.1.

part of hashicorp/terraform-provider-google#9611

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Release Note Template for Downstream PRs (will be copied)

composer: fixed environment version regexp to explicitly require . (dot) instead of any character after 'preview' (example: composer-2.0.0-preview.0-airflow-2.1.1)

Derived from GoogleCloudPlatform/magic-modules#5039