[Form] Fixed bug that incorrectly causes the "required" attribute to be ...

...omitted from select even though it contains the "multiple" attribute.

According to the HTML5 specification the required attribute should be allowed in this case: "A select element with a required attribute and **without** a multiple attribute, and whose size is “1”, must have a child option element."

Related PR: https://github.com/symfony/symfony/pull/9030

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT