[Php81] Skip non private modifier on ReadOnlyPropertyRector

[samsonasik]

Avoid error on public modifier like :

1) Rector\Tests\BetterPhpDocParser\PhpDocInlineHtml\TestInlineHtmlTest::test with data set #0 (Symplify\SmartFileSystem\SmartFileInfo Object (...))
Error: Cannot modify readonly property Rector\Core\PhpParser\Node\CustomNode\FileWithoutNamespace::$stmts

on #1364 (comment)

[samsonasik]

All checks have passed 🎉 @TomasVotruba it is ready for review.