Remove Ubuntu from CI workflow, using only macOS for testing (#868)

Author: meatball133

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-22.04, macos-15]
+        os: [macos-15]
         swift: ["6.0", "6.1"]
     env:
       RUNALL: "true"