CI: bump MacOS image to macos-12

The image is deprecated and we just suffered a brownout, upgrade before
we *have* to do so.

Author: Morwenn

.github/workflows/build-macos.yml

@@ -23,13 +23,13 @@ on:
 
 jobs:
   build:
-    runs-on: macos-11
+    runs-on: macos-12
 
     strategy:
       fail-fast: false
       matrix:
         cxx:
-          - g++-10
+          - g++-11
           - clang++
         config:
           # Release build