[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        | -
RequestStack didn't exist in 2.3.