Change platform from PHP 7.3 to 7.4

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

composer.json

@@ -34,7 +34,7 @@
 		"optimize-autoloader": true,
 		"classmap-authoritative": true,
 		"platform": {
-			"php": "7.3.0"
+			"php": "7.4.0"
 		}
 	}
 }