Drop Nextcloud 21 support

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

.github/workflows/php-test.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       matrix:
         php-versions: ['7.3', '7.4']
-        nextcloud-versions: ['stable21', 'stable22', 'stable23']
+        nextcloud-versions: ['stable22', 'stable23']
         include:
           - php-versions: '7.3'
             nextcloud-versions: 'stable20'

appinfo/info.xml

@@ -33,7 +33,7 @@
 	<screenshot>https://raw.githubusercontent.com/nextcloud/calendar/main/screenshots/week_sidebar.png</screenshot>
 	<dependencies>
 		<php min-version="7.3" max-version="8.1" />
-		<nextcloud min-version="21" max-version="25" />
+		<nextcloud min-version="22" max-version="25" />
 	</dependencies>
 	<background-jobs>
 		<job>OCA\Calendar\BackgroundJob\CleanUpOutdatedBookingsJob</job>