Modernize: PHP81 with rector

[kohlerdominik]

Inspired by #269, I took a dive whether there's some more code to modernize. I realized, that you already added Rector in #265, but it seems like either the 8.1 preset was updated since then, or it was not fully applied?

This PR

• simplifies and improves the rector config
  • importNames and importShortClasses config was kept
  • removeUnusedImports was added
  • withPhpSets uses the Composer config by default, so no explicit declaring of the set is required
• makes Output-examples more consistent and fixes, that Rector wants to remove the duplicate Sprain\SwissQrBill as QrBill import
• applies Rector for PHP81 set:
  • explicit (string) casts to address the deprecation of passing null to non-nullable parameters
  • apply newly introduced noUnusedImports fixer

[sprain]

Thank you! 🤩