Replace TargetedNativeComponent.targetPlatform() with targetPlatforms() in the API

- This change is to preserve compatibility from 2.1 -> 2.2
Remove generic BinarySpec parameter on ComponentSpec

- Simplified ComponentSpec.getBinaries() so it returns a Set<BinarySpec> (not <? extends BinarySpec>)
- Added NativeComponentSpec.getNativeBinaries() to provide a typed collection
- This should make it easier to migrate NativeComponentSpec to extend BaseComponentSpec
Update release notes for native API change