The behavior of `CameraOutputMode::Write { blend_state: None, .. }` is debatable

## Bevy version and features

main and before

## What you did

Multiple cameras output to the same render target, with default `CameraOutputMode::Write { blend_state: None, .. }`.

## What went wrong

The docs say that camera with `blend_state: None` will overwrite existing data:
https://github.com/bevyengine/bevy/blob/05ed7615632f2ced4efa394be103ec5fc543cabd/crates/bevy_camera/src/camera.rs#L819-L820

However the comment says the mode is alpha blending except the first camera:
https://github.com/bevyengine/bevy/blob/05ed7615632f2ced4efa394be103ec5fc543cabd/crates/bevy_core_pipeline/src/upscaling/mod.rs#L67-L70

I prefer to treat `blend_state: None` as disabling blend which is consistent with wgpu. Additionally we can change the default value to `ALPHA_BLENDING` to keep the behavior unchanged.

## Additional information

Posted by @beicause in https://github.com/bevyengine/bevy/pull/23422#issuecomment-4088849676:
>I think you're right from the intention of the original code. But the behavior of blend state None result in alpha blending seems a bit odd to me, at least in wgpu it means no blending.

>[tychedelia](https://github.com/tychedelia): I agree, and this relates to some of our previous discussions about multi-camera state. I would prefer if we did not have these implicit behaviors, or at least implicitly initialized some higher level configuration that controlled this.