Migrate lights to required components (#15554)

# Objective

Another step in the migration to required components: lights!

Note that this does not include `EnvironmentMapLight` or reflection
probes yet, because their API hasn't been fully chosen yet.

## Solution

As per the [selected
proposals](https://hackmd.io/@bevy/required_components/%2FLLnzwz9XTxiD7i2jiUXkJg):

- Deprecate `PointLightBundle` in favor of the `PointLight` component
- Deprecate `SpotLightBundle` in favor of the `PointLight` component
- Deprecate `DirectionalLightBundle` in favor of the `DirectionalLight`
component

## Testing

I ran some examples with lights.

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## Migration Guide

`PointLightBundle`, `SpotLightBundle`, and `DirectionalLightBundle` have
been deprecated. Use the `PointLight`, `SpotLight`, and
`DirectionalLight` components instead. Adding them will now insert the
other components required by them automatically.