Updated Scala.gitlab-ci.yml

The ci template for Scala projects points to a debian repository that was deprecated and is not available anymore in https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Scala.gitlab-ci.yml#L16

This updates the template according to the instructions in https://www.scala-sbt.org/1.x/docs/Installing-sbt-on-Linux.html#Ubuntu+and+other+Debian-based+distributions

(First contribution here. Sorry for not using a ticket template but I failed to find one that would fit, but hopefully the change is useful and simple enough to not require any 😀)