[Form] Moved POST_MAX_SIZE validation from FormValidator to request handler

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11729, #11877 
| License       | MIT
| Doc PR        | -
Exactly. If we disable the `$clearMissing` parameter, as in PATCH requests, that should work. I don't really want to add a way for marking a form as submitted, that could be misused quite heavily.