First Php8.6 Deprecation

[oleibman]

Mpdf uses soon-to-be-deprecated mb_regex_encoding.
https://wiki.php.net/rfc/eol-oniguruma

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests
• third-party deprecation

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary