Update return type for IntlDateFormatter::format()

Since PHP 8.0.0, the method is documented as having a `string|false` return (see https://github.com/php/php-src/blob/php-8.0.0/ext/intl/dateformat/dateformat.stub.php#L99-L104), this updates the polyfill to match the native signature which can help avoid static analysis issues where the polyfill has a wider return type than the native implementation.

Author: mbabker

src/Intl/Icu/IntlDateFormatter.php

@@ -203,7 +203,7 @@ public static function create(?string $locale, ?int $dateType, ?int $timeType, $
      *
      * @param int|string|\DateTimeInterface $datetime The timestamp to format
      *
-     * @return string|bool The formatted value or false if formatting failed
+     * @return string|false The formatted value or false if formatting failed
      *
      * @see https://php.net/intldateformatter.format
      *