Remove `extraErrorsBlockSubmit` flag on Form and make `extraErrors` always block validation

Prerequisites

• I have read the documentation

What theme are you using?

core

Is your feature request related to a problem? Please describe.

When the extraErrorsBlockSubmit flag was added back in the day, it was done to avoid a breaking change to how extraErrors affected the form submission.

Describe the solution you'd like

Given that users are likely to want them to block form submission, I recommend that we either remove the flag entirely OR change it to extraErrorsAreWarnings instead to switch the default behavior.

Describe alternatives you've considered

No response