Contact Form: move away from onSplit prop

[jeherve]

RichText’s optional onSplit prop was deprecated and replaced with a block.json support key called splitting.

We still use the prop here:

jetpack/projects/packages/forms/src/blocks/contact-form/components/jetpack-field-option.js

Line 53 in 40a8492

onSplit={ handleSplit }

Related PR: WordPress/gutenberg#54543

@monsieur-z already tackled related changes in #37739.