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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