This is a warning, not an error. It can be ignored or turned off so that it doesn't get logged.
The message means that the syntax of the code will be disallowed in a future version of php, not the version you are running.
The error is in the databridge mysql functions. MySQL will be discontinued entirely in a future version of php. You will need to update from MySQL to MySQLi to upgrade to php 7 or above and the file referenced in the error will no longer be used.