CI: use macos 12

Author: perazz

.github/workflows/CI.yml

@@ -21,14 +21,14 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-11, windows-latest]
+        os: [ubuntu-latest, macos-12, windows-latest]
         gcc_v: [10,11,12] # Version of GFortran we want to use.
         include:
         - os: ubuntu-latest
           os-arch: linux-x86_64
           release-flags: --flag '--static -g -fbacktrace -O3'
 
-        - os: macos-11
+        - os: macos-12
           os-arch: macos-x86_64
           release-flags: --flag '-g -fbacktrace -O3'