Update 403 error message.

The user should get a more friendly warning when their desktop client version is not supported anymore by the server.
See #nextcloud/desktop/issues/6273

Signed-off-by: Camila <hello@camilasan.com>

apps/dav/lib/Connector/Sabre/BlockLegacyClientPlugin.php

@@ -69,7 +69,7 @@ public function beforeHandler(RequestInterface $request) {
 		preg_match(IRequest::USER_AGENT_CLIENT_DESKTOP, $userAgent, $versionMatches);
 		if (isset($versionMatches[1]) &&
 			version_compare($versionMatches[1], $minimumSupportedDesktopVersion) === -1) {
-			throw new \Sabre\DAV\Exception\Forbidden('Unsupported client version.');
+			throw new \Sabre\DAV\Exception\Forbidden('This client is unsupported by the server: upgrade to the minimum required '.$minimumSupportedDesktopVersion.' version.');
 		}
 	}
 }