ci: switch from macOS 11 to macOS 12

The runner image for macOS 11 has been removed on 2024-06-28. See
<https://github.blog/changelog/2024-05-20-actions-upcoming-changes-to-github-hosted-macos-runners/>
for more information on that.

So the easiest fix is probably just to move to the next newest
version of macOS that is still avaiable for GHA.

.github/workflows/ci.yml

@@ -84,7 +84,7 @@ jobs:
             os: ubuntu-20.04
           - toolset: clang
             cxxstd: "14,17"
-            os: macos-11
+            os: macos-12
 
     runs-on: ${{matrix.os}}
     container: ${{matrix.container}}