Make compatible with scssphp 2.x

[malberts]

scssphp 1.x is now considered unmaintained as per scssphp/scssphp#145 (comment)

Release notes: https://github.com/scssphp/scssphp/releases/tag/v2.0.0

• this requires bumping minimum PHP >= 8.1
• Required for PHP 8.4 support

Not sure yet if this will cause issues for the Bootstrap extension which is still stuck on Bootstrap 4.6. For example:

• Switch from Sass imports to Sass Module System twbs/bootstrap#29853 (comment)

Reports of possible performance issues: scssphp/scssphp#785