[TODO] Use `truststore` in place of `ssl` by default

### Is your feature request related to a problem?

I mentioned this [once](https://github.com/aio-libs/aiohttp/issues/8430#issuecomment-2139581921) or [twice](https://github.com/aio-libs/aiohttp/issues/9296#issuecomment-2376954905) in the past. Now, I'm filing a tracking issue with action items.

People often face the problem of TLS certificate verification failing in the Python land while other tools in the same OS/runtime work. This is because Python's stdlib `ssl` is not set up to consult system trust stores.

The `truststore` library implements this with its drop-in replacement `SSLContext` and OS-specific API integrations. pip 24.2+ uses it by default and we should too.

This will improve the UX for our HTTP client. Though, the end-users can still use either stdlib `ssl`, or `truststore`-produce when passing an explicit context object.

### Describe the solution you'd like

Relying on system-managed TLS trust stores when making HTTPS requests.

Action items:
* [ ] locate all places constructing `ssl.SSLContext` objects (may be created via `ssl.create_default_context()`)
* [ ] replace those with `truststore.SSLContext`
* [ ] in runtime, prefer `truststore` which should be shielded on import with a fallback to just stdlib `ssl`
* [ ] `truststore` should probably be a mandatory runtime dependency in packaging core metadata; although, maybe we need to follow pip's example and make it optional first (via extras or manual install) and then add it unconditionally later
* [ ] document the priority and the compatibility considerations

### Describe alternatives you've considered

N/A

### Related component

Client

### Additional context

* https://truststore.rtfd.io
* https://pypi.org/p/truststore

### Code of Conduct

- [x] I agree to follow the aio-libs Code of Conduct