Minor version of Yii Strings package was released.

• Bump minimal required PHP version to 8.1 and minor refactoring
• Change PHP constraint in composer.json to ~8.1.0 || ~8.2.0 || ~8.3.0 || ~8.4.0
• Explicitly mark nullable parameters
• StringHelper::parsePath() for empty string returns path [''] instead of [] before
• Check string on a valid UTF-8 in StringHelper methods: trim(), ltrim() and rtrim()