Batch the triggerer's Trigger.clean_unused() cleanup deletes

### Under which category would you file this issue?

Providers

### Apache Airflow version

3.3.0.dev — current development branch

### What happened and how to reproduce it?

## Context

`airflow.models.trigger.Trigger.clean_unused()` is called every triggerer tick from `TriggererJobRunner._run_trigger_loop` and currently issues an unbounded `DELETE FROM trigger ...` (with a MySQL / non-MySQL fork for the `DELETE ... JOIN` problem). It has the same anti-pattern that the scheduler-side `SchedulerJobRunner._remove_unreferenced_triggers` was just fixed for: holds row locks on `trigger` for the full transaction and stalls the triggerer loop while many rows are removed.

Follow-up to #68241, which fixed the equivalent scheduler-side path and introduced the `[scheduler] unreferenced_triggers_cleanup_batch_size` config.

## Proposed fix

Apply the same LIMIT-bounded `select-IDs + delete-by-IDs + commit-between-batches` pattern that the scheduler-side fix uses, modelled on `airflow.utils.db_cleanup._do_delete` and `airflow.state.metastore.cleanup`. Either:

- Reuse the existing `[scheduler] unreferenced_triggers_cleanup_batch_size` config, or
- Add a triggerer-section twin (e.g. `[triggerer] unreferenced_triggers_cleanup_batch_size`) if reviewers prefer per-component tuning.

## Acceptance criteria

- The bulk `DELETE` in `Trigger.clean_unused()` is replaced by a batched loop.
- A unit test asserts multiple commits when the matching set exceeds the batch size.
- Existing `clean_unused` tests keep passing (semantics unchanged at default batch size).

## References

- AGENTS.md rule: [batched bulk DELETE/UPDATE in scheduler / interval callbacks](https://github.com/apache/airflow/blob/main/AGENTS.md#coding-standards)
- Template pattern: `airflow-core/src/airflow/utils/db_cleanup.py` — `_do_delete`
- Template pattern: `airflow-core/src/airflow/state/metastore.py` — `cleanup`

### What you think should happen instead?

This is a follow-up tracking issue (not a bug report). It captures deferred work from PR #68241, which fixed the scheduler-side `_remove_unreferenced_triggers` cleanup but intentionally left the parallel `Trigger.clean_unused()` path in the triggerer loop for a separate PR.

The triggerer's `airflow.models.trigger.Trigger.clean_unused()` runs on every triggerer tick and issues an unbounded `DELETE FROM trigger ...` (with a MySQL / non-MySQL fork for the `DELETE ... JOIN` problem). On busy installs this holds row locks on `trigger` for the full transaction and stalls the triggerer loop while many rows are removed — the same anti-pattern the AGENTS.md rule about batched bulk DELETE/UPDATE in scheduler/interval callbacks calls out.

### Operating System

N/A — not OS-specific (server-side scheduler/triggerer behaviour, applies to all deployments)

### Deployment

None

### Apache Airflow Provider(s)

_No response_

### Versions of Apache Airflow Providers

N/A — not provider-specific (issue is in airflow-core's triggerer cleanup path)

### Official Helm Chart version

Not Applicable

### Kubernetes Version

_No response_

### Helm Chart configuration

_No response_

### Docker Image customizations

_No response_

### Anything else?

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)