Just to echo what Dan is saying here - in theory you could go carefully through the code and apply all the relevant fixes from the latest version of Moodle. But that is a) going to involve a lot more time and effort than simply upgrading to the latest version of Moodle b) not going to address any other security fixes that you are missing and c) likely to introduce bugs that you will need to resolve (as the fixes may rely on bits of code that are present in the later versions of Moodle, but not in your version and they may also use bits of syntax that are only available in PHP 7.0 and above).
Upgrading Moodle isn't usually that complicated to do - it will require upgrading to a higher version of PHP and deploying a couple of intermediate versions of Moodle (as you can't jump direct from 2.7 to 3.9), but it's going to be a lot less painful than all your users having to deal with these error messages.