Modernize CI

- Do not test against Ruby 2.1 and 2.2
- Test against latest Ruby versions
- Fix actions permissions for better security
- Update GitHub actions
- Add DependabBot

.github/dependabot.yml

@@ -0,0 +1,11 @@
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+  - package-ecosystem: bundler
+    directory: /
+    schedule:
+      interval: weekly
+    open-pull-requests-limit: 10

.github/workflows/ci.yml

@@ -6,23 +6,26 @@ on:
   pull_request:
     branches: [ master ]
 
+permissions:
+  contents: read
+
 jobs:
   test:
     name: Specs
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['2.1', '2.2', '2.3', '2.4', '2.5', '2.6', '2.7', '3.0']
-        experimental: [false]
+        ruby-version: ['2.3', '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3']
+        channel: ['stable']
 
         include:
           - ruby-version: 'head'
-            experimental: true
+            channel: 'experimental'
 
-    continue-on-error: ${{ matrix.experimental }}
+    continue-on-error: ${{ matrix.channel != 'stable' }}
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with: