[WIP] Upgrade gitamp to use latest amphp packages

[ekinhbayar]

This is rather a complete overhaul touching every single file...

• PHP version bumped to 7.4, now the properties are typed \o/
• PHPUnit version bumped to ^9
• Dropped username+password authentication for GitHub API, since it's being deprecated
• Aerys and Artax replaced with HttpServer and Client
• Websocket handler and really all things amphp refactored to use latest amphp packages available for this
• All tests refactored
• Dropped travis nightly build because it doesn't seem to work at all
• Dropped 7.2, 7.3 builds since we have typed properties now and PHPUnit ^9 requires minimum 7.3 anyway

To-Do

• [x] .editorconfig
• [x] Dependency checker