Set proper public webdav permissions when public upload disabled

Fixes #23325

It can happen that a user shares a folder with public upload. And some
time later the admin disables public upload on the server.

To make sure this is handled correctly we need to check the config value
and reduce the permissions.

Fix is kept small to be easy backportable.

apps/dav/lib/connector/publicauth.php

@@ -61,6 +61,11 @@ class PublicAuth extends \Sabre\DAV\Auth\Backend\AbstractBasic {
 			return false;
 		}
 
+		if ((int)$linkItem['share_type'] === \OCP\Share::SHARE_TYPE_LINK &&
+			$this->config->getAppValue('core', 'shareapi_allow_public_upload', 'yes') !== 'yes') {
+			$this->share['permissions'] &= ~(\OCP\Constants::PERMISSION_CREATE | \OCP\Constants::PERMISSION_UPDATE);
+		}
+
 		// check if the share is password protected
 		if (isset($linkItem['share_with'])) {
 			if ($linkItem['share_type'] == \OCP\Share::SHARE_TYPE_LINK) {