If `--ios_simulator_device` is not specified, the value defaults to iPhone 6s Plus, which is not useful as of 2025.

Context: https://github.com/google/google-toolbox-for-mac/pull/539

Although the code in https://github.com/google/google-toolbox-for-mac/actions/runs/17138256361/job/48622207526 suggests:

```starlark
device_type = ctx.attr.device_type or ctx.fragments.objc.ios_simulator_device or "iPhone 15"
```

In practice, the value of `device_type` falls back to **iPhone 6s Plus** when `--ios_simulator_device` is not specified. Since the iPhone 6s Plus is a very old device, we generally don’t expect its iOS Simulator runtime to be installed anymore.

I’m curious where this default of **iPhone 6s Plus** originates, and it would be better to replace it with a more reasonable default rather than such an outdated choice.

Start by tracing the device_type fallback shown in the issue and compare it with the --ios_simulator_device configuration, using the linked pull request and action run as context. Identify where iPhone 6s Plus is introduced, replace that outdated default with the agreed useful value, and verify that simulator selection works when the flag is omitted.

Written by the indexing model from the issue text.