Change development flow to use stable branches

[vitormattos]

Background

This version of Nextcloud requires at least PHP 7.4<br/>You are currently running 7.3.33-1+ubuntu20.04.1+deb.sury.org+1. Please update your PHP version.

Originally posted by @rotdrop in #481 (comment)

Motivation

Maintain the latest version of app compatible with the improvements from the newest server version.

To do

• Create stable branches to actual supported versions: stable22, stable23, stable24
  Suggestion:

  Server version	Repository branch	app version	PHP
  22	stable22	2.x.x	7.3
  23	stable23	3.x.x	7.3
  24	stable24	4.x.x	7.4
  25	master (will be stable25)	5.x.x	7.4

• Change info.xml on stable branches to change the min and max version
• Remove the platform->php with the PHP version from composer.json (need to follow the min version of stable branch)
• Change the server version and PHP version on .github/workflows/phpunit.yml
• Launch new releases