Deprecate implicit legacy SQL default in BigQuery operators

[VladaZakharova]

Deprecates the implicit use_legacy_sql=True default in older BigQuery operators.

BigQuery now recommends GoogleSQL, and legacy SQL availability is restricted after June 1, 2026 depending on prior usage. This change keeps the current runtime behavior for compatibility, but emits an AirflowProviderDeprecationWarning when use_legacy_sql is omitted and Airflow falls back to the old legacy SQL default.
Documentation: https://docs.cloud.google.com/bigquery/docs/legacy-sql-feature-availability#:~:text=BigQuery%20monitors%20the%20use%20of,after%20the%20evaluation%20period%20ends.

Users can avoid the warning by choosing explicitly:

use_legacy_sql=False to use GoogleSQL.
use_legacy_sql=True if they intentionally still need legacy SQL.
The warning applies only to the implicit default. Explicit legacy SQL usage remains supported.

This also updates BigQuery operator and connection docs with guidance about GoogleSQL and legacy SQL availability.

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Important

🛠️ Maintainer triage note for @VladaZakharova · by @potiuk · 2026-06-17 18:53 UTC

Some review feedback from shahar1 is waiting on you:

• Please respond to the open review thread(s) — reply or push a fix in each, then mark them resolved.
• See the Pull Request quality criteria.

The ball is in your court — you've been assigned to this PR. Reply or push a fix in each thread, then mark them resolved.

_{Automated triage — may be imperfect; a maintainer takes the next look.}

[potiuk]

@VladaZakharova A few things need addressing before review — see our Pull Request quality criteria.

• ❌ CI fails: MySQL tests: core / DB-core:MySQL:8.0:3.10:API...CLI (and possibly other checks — see the Checks tab for the full list).

No rush.

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Note: This comment was drafted by an AI-assisted triage tool and may contain mistakes. Once you have addressed the points above, an Apache Airflow maintainer — a real person — will take the next look at your PR. We use this two-stage triage process so that our maintainers' limited time is spent where it matters most: the conversation with you.

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Drafted-by: Claude Code (Opus 4.7); reviewed by @potiuk before posting

[shahar1]

Mypy fail seems unrelated