dep-dev: bump ruby from 2.7.5-slim-bullseye to 3.1.1-slim-bullseye in /.gitlab/docker

Can't show this merge request because the source branch dependabot-docker-.gitlab-docker-ruby-3.1.1-slim-bullseye is missing from project georgekoltsov-multiple-projects-test-4/imported-project-ef029319134ab099. Close this merge request or update the source branch.