Remove the `load_folder` API

Originally posted by @andriyDev in https://github.com/bevyengine/bevy/issues/19717#issuecomment-2985476880:

@alice-i-cecile I am against the `load_folder` API. I think my ideal API is something like this:

```rust
struct LoadBatchRequest {
requests: Vec,
}
```

So you could do:
```
vec![
"my_mesh.gltf",
"animations/walk_*.png",
"animations/run_*.png",
"particles/*",
]
```

We can even make this itself an asset, so you'd just do `asset_server.load::("my_load_request.batch.ron");`. I think this allows us to be more targeted and is generally more flexible. You can still do folder loads, but you can also target specific file paths.

I would prefer not to merge this.

Start by locating the `load_folder` API and all of its callers, then review the linked discussion for the intended replacement direction. Done means the API is removed and the repository remains consistent, with affected tests or examples updated as needed.

Written by the indexing model from the issue text.