Upgrade Ruby to version 3.0.6

Note the structure of the Gemfile changes slightly because Ruby 3.0
includes a new version of the Bundler gem.

.gitlab-ci.yml

@@ -3,7 +3,7 @@ stages:
   - lint
 
 tests:
-  image: "ruby:2.7.7"
+  image: "ruby:3.0.6"
   stage: test
   services:
     - postgres:10.10
@@ -34,7 +34,7 @@ tests:
 # PRONTO_GITLAB_API_PRIVATE_TOKEN to your repository CI/CD settings
 # giving it the value of the Personal Access Token
 linters:
-  image: "ruby:2.7.7"
+  image: "ruby:3.0.6"
   stage: lint
   cache:
     key: consul