Form submission doesn't go through Form::saveInto()

[GuySartorelli]

#1265 highlighted that the data submitted through the elemental area form doesn't get saved into elements using the standard Form::saveInto(). Instead, similar (but less complex) logic is re-implemented inside BaseElement::updateFromFormData().

We should rely on the standard form submission flow as much as possible, which means removing BaseElement::updateFromFormData() in favour of Form::saveInto()

Acceptance Criteria

• Form submissions for elemental blocks get saved into elements using Form::saveInto() instead of custom logic

CMS 5 PRs

• API Deprecate API which will be removed #1270
• DOC Document API that is being removed developer-docs#624

After merging, reassign to Guy to rebase the CMS 6 PR.

CMS 6 PRs

• API Remove custom logic in favour of Form::saveInto() #1269