Update version.php to owncloud 10.13

ownCloud's current version is 10.13
migration was tested with a customer; 
documentation was adjusted
https://github.com/nextcloud/documentation/pull/11225

Signed-off-by: Rello <Rello@users.noreply.github.com>

version.php

@@ -41,7 +41,7 @@ $OC_VersionCanBeUpgradedFrom = [
 		'28.0' => true,
 	],
 	'owncloud' => [
-		'10.11' => true,
+		'10.13' => true,
 	],
 ];