Batch triggerer unused trigger cleanup deletes

[BeauDevCode]

closes: #68243

Why

Trigger.clean_unused() currently removes all unreferenced trigger rows in one unbounded delete. On busy deployments this can hold locks on the trigger table for the duration of a large cleanup and stall the triggerer loop.

What changed

• Select unused trigger IDs before deleting, preserving the existing unused-trigger criteria.
• Delete matching trigger rows in configurable batches, committing between batches.
• Add [triggerer] unreferenced_triggers_cleanup_batch_size, defaulting to 500; setting it to 0 keeps single-transaction cleanup behavior.
• Add a unit test that uses a batch size of 2 and verifies five unused triggers are removed across three committed delete batches.

Related context: PR #68244 changes the anti-join shape in the same method, but does not add bounded delete batches. This PR intentionally focuses on the batching behavior requested in #68243.

Tests

• git diff --check
• python3 -m compileall -q airflow-core/src/airflow/models/trigger.py airflow-core/tests/unit/models/test_trigger.py
• Parsed airflow-core/src/airflow/config_templates/config.yml with PyYAML and verified the new option shape

Not run locally: focused pytest/config validation could not complete because this environment lacks parts of Airflow's development dependency set (pathspec during test collection; requests for dev/validate_version_added_fields_in_config.py).