Remove Ruby 2.6 from test matrixes

.github/workflows/ruby.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['2.6', '2.7', '3.0', '3.1']
+        ruby-version: ['2.7', '3.0', '3.1']
 
     steps:
     - uses: actions/checkout@v3

.gitlab-ci.yml

@@ -5,7 +5,7 @@ cache:
     - vendor/ruby
 
 lint:rubocop:
-  image: "ruby:2.6"
+  image: ruby
   script:
     - gem install rubocop -N
     - rubocop lib
@@ -25,10 +25,6 @@ lint:rubocop:
     reports:
       junit: "reports/TEST-*.xml"
 
-test:ruby2.6:
-  extends: .test
-  image: "ruby:2.6"
-
 test:ruby2.7:
   extends: .test
   image: "ruby:2.7"