Drop CI for MacOS Julia 1.5

The latest MacOS is not compatible with x86 processors, and Julia 1.5 is only
compatible with x86. We could test on older versions of MacOS, but why bother.

.github/workflows/UnitTests.yml

@@ -12,16 +12,19 @@ jobs:
       fail-fast: false
       matrix:
         julia-version:
-          - '1.5' # Minimally supported version of Julia
+          - '1.5' # Minimally supported version of Julia.
           - '1'
         julia-arch: [x86]
-        os: [ubuntu-latest, windows-latest, macOS-latest]
+        os: [ubuntu-latest, windows-latest]
         experimental: [false]
         include:
           - julia-version: nightly
             julia-arch: x86
             os: ubuntu-latest
             experimental: true
+          - julia-version: 1
+            os: macOS-latest
+            experimental: false
 
     steps:
       - name: Checkout Repository