TypeHints.DNFTypeHintFormat shortNullable should be "yes" for PHP < 8

[kkmuffme]

SlevomatCodingStandard.TypeHints.DNFTypeHintFormat

https://github.com/slevomat/coding-standard/blob/master/doc/type-hints.md#slevomatcodingstandardtypehintsdnftypehintformat-

shortNullable: yes requires usage of ? for nullable type hint, no disallows it. None is set by default so both are enabled.

Union types are only supported from PHP 8.0+, therefore it should always be set to "yes" by default for lower versions.

This allows a config that uses "no" (or no value set) to be usable for PHP versions from PHP 7.1-8.4 without causing a fatal error for nullable types