Simplify the README

Author: eregon

README.md

@@ -48,6 +48,8 @@ jobs:
 
 ### Matrix
 
+This matrix tests all stable releases of MRI, JRuby and TruffleRuby on Ubuntu and macOS.
+
 ```yaml
 name: My workflow
 on: [push]
@@ -57,7 +59,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ ubuntu-latest, macos-latest ]
-        ruby: [ 2.6, 2.7, jruby, truffleruby ]
+        ruby: [ 2.4, 2.5, 2.6, 2.7, jruby, truffleruby ]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2
@@ -74,29 +76,6 @@ jobs:
 * version only like `2.6.5`, assumes MRI for the engine
 * engine only like `truffleruby`, uses the latest stable release of that implementation
 
-### All Stable Versions
-
-With that, we can test on all stable releases of MRI, JRuby and TruffleRuby with:
-
-```yaml
-name: My workflow
-on: [push]
-jobs:
-  test:
-    strategy:
-      fail-fast: false
-      matrix:
-        os: [ ubuntu-latest, macos-latest ]
-        ruby: [ 2.4, 2.5, 2.6, 2.7, jruby, truffleruby ]
-    runs-on: ${{ matrix.os }}
-    steps:
-    - uses: actions/checkout@v2
-    - uses: eregon/use-ruby-action@master
-      with:
-        ruby-version: ${{ matrix.ruby }}
-    - run: ruby -v
-```
-
 ## Limitations
 
 * Currently does not work on Windows since the builder doesn't build on Windows.