[Form] Changed "allow_html5" to "html5"

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This is a follow-up to #8291, which I missed before it was merged. IMO "allow_html5" is a confusing name. We don't "allow" HTML5, but we use it - or we don't. I think "html5" conveys that meaning well and is shorter at the same time.