Deprecate `AssetId::invalid()` (#24392)

## Objective

Progress #19024 (removing UUID handles). Even if UUID handles end up
sticking around, there's an argument for deprecating
`AssetId::invalid()` in the name of simplicity and robustness.

## Solution

Remove the last remaining case of `AssetId::invalid()` in the
`custom_phase_item` example. The example only used it as a placeholder
value, so `AssetId::default()` is fine.

```diff
 Opaque3dBinKey {
-    asset_id: AssetId::<Mesh>::invalid().untyped(),
+    asset_id: AssetId::<Mesh>::default().untyped(),
 }
```

## Testing

```sh
cargo run --example custom_phase_item
```