Remove deprecated macOS 12 CI build (#1905)

Unfortunate, but until I can get access to a macOS machine we'll have to live without macOS CI as my previous attempt at adding macOS 13 CI is failing.

.github/workflows/main.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-22.04, macos-12, windows-2022]
+        os: [ubuntu-22.04, windows-2022]
         platform: [x64]
         build-cfg: [DebugOpt, Release]
         #build-cfg: [Debug, DebugOpt, Release] # our local copy of clang isn't build for debug on linux/macos currently