Breaking: #69401 - Adopt ext:form to support the Extbase/ Fluid MVC stack

See Issue #69401

Description

The postProcessor interface and the mail postProcessor have changed.

Validators and filters have been moved to other folders and both class names and algorithms have changed.

Impact

Own postProcessors, validators and filters will possibly fail with an error.

Affected Installations

Installations with own postProcessors, validators and filters.

Migration

Adopt own postProcessors, validators and filters to comply with the current implementation.