Add support for Composer maintenance window configuration

[modular-magician]

Add maintenance_window to google_composer_environment (beta)

fixes #9423

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• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• 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: added field `maintenance_window` to resource `google_composer_environment` (beta)

Derived from GoogleCloudPlatform/magic-modules#4961

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