Allow Symfony 8 + streamline docker scripts

Project cleanup + strings dependency upgrade

Allow Symfony 7 + require PHP 8.1

Relax underlying library version constraint

Support new RemoveNonAlphabetic filter

Bump debug dependency

Fix annotation mentions in documentation

Set minim PHP version to PHP8

- Adapt to new Attribute directory instead of the
  previous Annotation directory from underlying
  strings library (BC break)
- Cover tests with Psalm

Add StreamlineEmail filter as a service

Add PHP8 attributes to documentation

- Update dependencies
- Test bundle in PHP8
- Adjust configuration