I did this a while ago for myself; it looks like the setting can be found here[1], labelled as a checkbox "Keep my email addresses private". Note that (I think, not sure if it gets overridden if you check the box), you also need to set your "user.email" git config to use the noreply email they provide. Personally, I'd never committed to an external 3rd-party repository, and I crushed/re-imported (erasing history) my own repositories using the new noreply email. This is because I know no external users were depending on my old repositories. I suspect that if you've already pushed commits to a 3rd-party repo, it's too late (the email address is part of the commit and cannot be revoked without a history-changing rebase). In my case, I deleted my repos and re-imported, history be damned.
Ah thanks - yeah I've already pushed with my new aliased email for git (I recently switched to Fastmail and set up a less extreme alias configuration :) ).
[1] https://help.github.com/en/github/setting-up-and-managing-yo...