build(CI): update GCC from 11 to 12

Author: rouson

.github/workflows/fpm.yml

@@ -9,7 +9,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest]
-        gcc_v: [11] # Version of GFortran we want to use.
+        gcc_v: [12] # Version of GFortran we want to use.
         include:
         - os: ubuntu-latest
           os-arch: linux-x86_64