Documentation is outdated in HACKING/GettingStarted.md

While handling #33741 (closed), I saw in the mentioned file that the documentation states :

8. Submitting your patch                                                           
                                                                                   
   We review patches through tickets on our bugtracker at                          
  [trac](https://trac.torproject.org). You can either upload your patches there, or
   put them at a public git repository somewhere we can fetch them                 
   (like gitlab, github or bitbucket) and then paste a link on the appropriate     
   trac ticket.                                                                    

This is not true anymore as we use Gitlab. I can modify this to give the gitlab link and add a sentence saying that if users want to use gitlab, they need to ask for an account.

I will then make two separated MR for #33741 (closed) and this issue.