React would treat onSubmit as same as action prop

[ryota-murakami]

Support useFormStatus in progressively-enhanced forms facebook/react#29019
In this PR, Andrew says

This is the last missing piece for implementing a custom action prop that is progressively enhanced using onSubmit while maintaining > the same behavior as built-in form actions.

This mean,
I guess onSubmit handler can use as Server Actions.
I'm not good at technical detail though.