Test Julia 1.6 on x86 macOS instead of ARM

See: https://discourse.julialang.org/t/how-to-fix-github-actions-ci-failures-with-julia-1-6-or-1-7-on-macos-latest-and-macos-14/117019

.github/workflows/ci.yml

@@ -34,7 +34,14 @@ jobs:
           - false
         os: [ubuntu-latest, windows-latest, macos-latest]
         arch: [x64]
+        exclude:
+          # No native binaries for Julia 1.6 on ARM macOS
+          - os: macos-latest
+            version: '1.6'
         include:
+          # In its place, test Julia 1.6 on x86 macOS
+          - os: macos-13
+            version: '1.6'
           - os: ubuntu-latest
             experimental: false
             prefix: xvfb-run  # julia-actions/julia-runtest/blob/master/README.md