[Horizon] [stable] Adding Radomir Dopieralski to horizon-stable-maint

Hi team, I'd like to propose adding Radomir Dopieralski to the horizon-stable-maint team. He's doing good quality reviews for stable branches [1] on a regular basis and I think Radomir will be a good member of our small group. [1] https://review.opendev.org/#/q/reviewer:openstack%2540sheep.art.pl+NOT+branc... Regards, Ivan Kolodyazhny, http://blog.e0ne.info/