feat: allow ExplicitNullableParamTypeRector to run for PHP 8.1

This allows this rule to be applied one minor version earlier in "eager" mode,
since DNF type specifications came one step after intersection types
themselves.

I was originally just going to bump the required PHP version up to 8.2, but
I saw precedence for adding version guards in other rules:

- ReturnTypeFromStrictTypedCallRector
- AddParamTypeDeclarationRector
- IsIterableRector

So when PHP 8.1 is a possibility, we can still *reduce* the number of
deprecations without fully committing to them all.

Author: Kenneth-Sills

rules/Php84/Rector/Param/ExplicitNullableParamTypeRector.php

@@ -93,6 +93,10 @@ public function refactor(Node $node): ?Param
         if ($node->type instanceof UnionType) {
             $node->type->types[] = new Name('null');
         } elseif ($node->type instanceof ComplexType) {
+            if (! $this->phpVersionProvider->isAtLeastPhpVersion(PhpVersionFeature::UNION_INTERSECTION_TYPES)) {
+                return null;
+            }
+
             /** @var IntersectionType $nodeType */
             $nodeType = $node->type;
             $node->type = new UnionType([$nodeType, new Name('null')]);