build(CI): switch to macos-latest but keep GCC 10

Author: rouson

.github/workflows/fpm.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-11]
+        os: [ubuntu-latest, macos-latest]
         gcc_v: [10] # Version of GFortran we want to use.
         include:
         - os: ubuntu-latest