ci: remove macos-12

it has been deprecated in actions/runner-images#10721

Author: dvirtz

.github/workflows/main.yml

@@ -12,7 +12,6 @@ jobs:
         os:
           - ubuntu-20.04
           - windows-2019
-          - macos-12
           - macos-14
     env:
       CONAN_HOME: "${{ github.workspace }}/conan-cache"