Drop old MacOS jobs: there are no bindists of old GHCs for aarch64, which...

Drop old MacOS jobs: there are no bindists of old GHCs for aarch64, which macos-latest now refers to

.github/workflows/ci.yml

@@ -17,6 +17,17 @@ jobs:
       matrix:
         os: [ubuntu-22.04, macOS-latest]
         ghc: ['9.8', '9.6', '9.4', '9.2', '9.0', '8.10', '8.8', '8.6']
+        exclude:
+          - os: macos-latest
+            ghc: '9.0'
+          - os: macos-latest
+            ghc: '8.10'
+          - os: macos-latest
+            ghc: '8.8'
+          - os: macos-latest
+            ghc: '8.6'
+          - os: macos-latest
+            ghc: '8.4'
     steps:
     - uses: actions/checkout@v4
     - name: Setup toolchain