Support rails 6.1+, ruby 2.7+ in test

Rails 7 and 7.1 still support ruby 2.7 so we set this ruby as the minimum we test,
keep rails 6.1 as it's not yet end of life and use bundler 2.4, which is the last
version which supports ruby 2.7.

.github/workflows/testing.yml

@@ -18,8 +18,8 @@ jobs:
       - name: Setup ruby
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 2.1
-          bundler: 1.17
+          ruby-version: 2.7
+          bundler: 2.4.22
           bundler-cache: true
 
       - name: Run rubocop
@@ -34,44 +34,14 @@ jobs:
 
       matrix:
         ruby:
-          - 2.1
-          - 2.5
           - 2.7
           - 3.0
           - 3.1
           - 3.2
         rails:
-          - 3.2
-          - 4.2
-          - 5.2
           - 6.1
           - 7.0
-        exclude:
-          - rails: 5.2
-            ruby: 2.1
-          - rails: 6.1
-            ruby: 2.1
-          - rails: 7.0
-            ruby: 2.1
-          - rails: 7.0
-            ruby: 2.5
-          - rails: 3.2
-            ruby: 2.7
-          - rails: 4.2
-            ruby: 2.7
-          - rails: 3.2
-            ruby: 3.0
-          - rails: 4.2
-            ruby: 3.0
-          - rails: 3.2
-            ruby: 3.1
-          - rails: 4.2
-            ruby: 3.1
-          - rails: 3.2
-            ruby: 3.2
-          - rails: 4.2
-            ruby: 3.2
-
+          - 7.1
     steps:
       - name: Checkout source
         uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4
@@ -80,7 +50,7 @@ jobs:
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
-          bundler: 1.17
+          bundler: 2.4.22
           bundler-cache: true
         env:
           RAILS_VERSION: ${{ matrix.rails }}