Fix composer environment version regexp

[gor-obr]

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

If this PR is for Terraform, I acknowledge that I have:

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• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
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• Read the Release Notes Guide before writing my release note below.

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)

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I have detected that you are a community contributor, so your PR will be assigned to someone with a commit-bit on this repo for initial review.

Thanks for your contribution! A human will be with you soon.

@rileykarson, please review this PR or find an appropriate assignee.

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
Terraform Beta: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))

[rileykarson]

Do you mind rebasing? Sorry, seems there's been a flake in CI that should be resolved by taking in #5038

[melinath]

This shouldn't require rebasing - /gcbrun should be enough

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
Terraform Beta: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))

[rileykarson]

/gcbrun

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
Terraform Beta: Diff ( 3 files changed, 3 insertions(+), 4 deletions(-))

[rileykarson]

Sorry for the delay in getting back! I need to declare an email bankruptcy, or something.