ci: only test native_ext on ubuntu

Author: p0deje

.github/workflows/ci.yml

@@ -109,18 +109,14 @@ jobs:
 
   examples-native_ext:
     name: Examples / Native Ext
-    runs-on: ${{ matrix.os }}
+    runs-on: ubuntu-latest
     timeout-minutes: 30
     strategy:
       fail-fast: false
       matrix:
         ruby:
           - 3.3.6
           - jruby-9.4.9.0
-        os:
-          - ubuntu-latest
-          - macos-latest
-          - windows-latest
     defaults:
       run:
         working-directory: examples/native_ext

examples/native_ext/.ruby-version

@@ -1 +1 @@
-3.3.6
+jruby-9.4.9.0