[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        | -
Yes, there's a lot of similarities between the two request handlers. On the other hand, there are lots of tiny differences, so we'd end up with a pretty complex class + control flow. Both classes are covered by the same test case, so if tests are added, both classes need to be adapted.

Feel free to try to remove the duplication in a new PR, there we can discuss how to do that well and whether it is really worth it.