[Form] Add a form error if post_max_size has been reached.
[Form] Moved POST_MAX_SIZE validation from FormValidator to request handler
bug #11924 [Form] Moved POST_MAX_SIZE validation from FormValidator to request handler (rpg600, webmozart)

This PR was merged into the 2.3 branch.

Discussion
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[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        | -

Commits
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759ae1a [Form] Moved POST_MAX_SIZE validation from FormValidator to request handler
4780210 [Form] Add a form error if post_max_size has been reached.