CI: Stop running on macOS

Author: amotl

.github/workflows/tests.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ ubuntu-latest, macos-latest ] # , windows-latest ]
+        os: [ ubuntu-latest ] # macos-latest, windows-latest
         ruby-version: [ "2.6", "2.7", "3.0", "3.1" ]
 
     services: