Suggestion: Introduce some enums (requires bumping minimum PHP version to 8.1)

I just stumbled over [`Dompdf::$paperOrientation`](https://github.com/dompdf/dompdf/blob/master/src/Dompdf.php#L116) and thought about submitting a PR to replace the string with a `PaperOrientation` enum.

What's your plan regarding the minimum PHP version? Right now, you're still supporting 7.1, which reached EOL in 2019: https://www.php.net/eol.php