Clean up .gitlab-ci.yml

Make use of parallel and matrix builds.

Note Ruby v3.1 is not yet supported due to a breakage of `mail` gem
due to a missing `net-smtp` dependency:
mikel/mail#1439 (comment)

.gitlab-ci.yml

@@ -1,4 +1,5 @@
-# Cache gems in between builds
+default:
+  image: "ruby:${RUBY_VERSION}"
 
 services:
   - redis:latest
@@ -9,24 +10,17 @@ services:
       - vendor/ruby
   variables:
     REDIS_URL: redis://redis:6379
-  script:
-    - bundle exec rspec spec
   before_script:
     - apt update && apt install -y libicu-dev
     - ruby -v                                   # Print out ruby version for debugging
-    # Uncomment next line if your rails app needs a JS runtime:
-    # - apt-get update -q && apt-get install nodejs -yqq
-    - gem install bundler  --no-document    # Bundler is not installed with the image
-    - bundle install -j $(nproc) --path vendor  # Install dependencies into ./vendor/ruby
-
-rspec-2.6:
-  image: "ruby:2.6"
-  <<: *test
-
-rspec-2.7:
-  image: "ruby:2.7"
-  <<: *test
+    - gem install bundler --no-document         # Bundler is not installed with the image
+    - bundle config set --local path 'vendor'
+    - bundle install -j $(nproc)
+  script:
+    - bundle exec rspec spec
 
-rspec-3.0:
-  image: "ruby:3.0"
+rspec:
+  parallel:
+    matrix:
+      - RUBY_VERSION: [ "2.5", "2.6", "2.7", "3.0" ]
   <<: *test