Add ga version of bigquery reservation

[modular-magician]

https://cloud.google.com/bigquery/docs/release-notes#April_16_2020

Moves bigqueryreservation to GA.

Fixes #8058.

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• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

bigquery: promoted bigquery reservation to GA.

Derived from GoogleCloudPlatform/magic-modules#4342

[ghost]

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