Migrate dev.gitlab.org to use Let's Encrypt

Currently, dev.gitlab.org certificate is issued by SSLMate, which requires us to go through a somehow-long manual procedure (e.g. production#5871 (closed)) to update the certificate after renewal. Switch to Let's Encrypt allows us to use certbot for automatic renewal so we don't need to worry about renewals ourselves.

Setting the priority and severity to 3 as we are covered for the year, but come next year we would like to not go through the manual process again.