Postgres: Make psycopg (v3) the default synchronous Postgres driver

[Dev-iL]

related:

• Remove psycopg2 and migrate the sync Postgres driver to psycopg3 #68453
• Switch the default async Postgres driver from asyncpg to psycopg3 (provider) #69089

Mirrors the async default switch: PostgresHook no longer requires psycopg2 at import time. psycopg2-binary moves from a hard dependency to a new [psycopg2] optional extra.

What changed

• providers/postgres/pyproject.toml: psycopg2-binary moved from dependencies into a new "psycopg2" optional-dependencies extra, with its existing version constraints preserved. The apache-airflow>=2.11.0 floor and the "sqlalchemy" extra are unchanged. uv.lock regenerated.
• providers/postgres/src/airflow/providers/postgres/hooks/postgres.py: the psycopg2 connection/cursor/execute_values imports are now lazy/guarded (same try/except pattern already used for the psycopg3 import). The module imports successfully even when psycopg2 isn't installed, and any call site that genuinely needs the psycopg2 path raises a clear AirflowOptionalProviderFeatureException instead of failing at import time. USE_PSYCOPG3/is_sqla2 dual-path logic is otherwise unchanged.
• providers/postgres/README.rst / docs/index.rst: regenerated to include the new [psycopg2] extra.
• providers/postgres/docs/changelog.rst: new "Breaking changes" note describing the sync default change and the remediation (pip install apache-airflow-providers-postgres[psycopg2]).

Test plan

• New/updated tests in providers/postgres/tests/unit/postgres/hooks/test_postgres.py simulate psycopg2 being absent (sys.modules patching) and confirm the module still imports, while each of the three guarded call sites (_get_cursor, _create_connection, insert_rows(fast_executemany=True)) raises the clear, actionable error rather than a bare traceback.

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