Nextcloud 20 compatibility

[R0Wi]

This app should work with NC 20.

• Update appinfo/info.xml
• Update README.md
• Update test actions in .github/workflows to run on stable20
• Remove appinfo/app.php and appinfo/autoload.php and use IBootstrap instead
• Deprecate \OCP\ILogger
• Deprecate \OCP\AppFramework\IAppContainer and \OCP\IServerContainer
• Deprecate \OCP\IServerContainer::getEventDispatcher
• Cover Appinfo/Application.php
• Cover Eventlistener for registration or exclude from coverage
• Check autoload functionallity for vendor after removing autoload.php

More info at https://docs.nextcloud.com/server/latest/developer_manual/app_publishing_maintenance/upgrade-guide.html