Evaluate Weblate's use of SSH keys and proceed accordingly

After the upgrade to Weblate 3.8, we started receiving such notices over e-mail:

## Could not update the repository.                                                                                                                                                            
                                                                                                                                                                                               
Weblate could not fetch upstream changes when updating the repository.                                                                                                                         
                                                                                                                                                                                               
    no such identity: /var/lib/weblate/repositories/ssh/id_rsa: No such file or directory                                                                                                      
    Permission denied (publickey).                                                                                                                                                             
    fatal: Could not read from remote repository.                                                                                                                                              
                                                                                                                                                                                               
    Please make sure you have the correct access rights                                                                                                                                        
    and the repository exists. (128)  

A key was created using Weblate's web interface and the notifications stopped, but we're not sure what that key should/could be used for and if we need to manage its configuration with Puppet.