v1.6.0 General Maintenance: dependencies et al.

What's Changed

• Dependencies
  • added: "psr/http-message": "^1.1 || ^2.0" as a direct dependency!
  • removed: support for guzzlehttp/psr7 < 2.4.5
  • bumped to webmozart/assert:^1.11
• Running CI checks fro PHP 7.4 - 8.3
• Minor code fixes, due to psalm reports
• Update dev dependencies
• Update CI (GH actions)
• Fix local dev env / dockerfile
• Update code style and cs-fixer

Full Changelog: v1.5.0...v1.6.0