Update CI

Author: jchristopherson

.github/workflows/fpm.yml

@@ -9,7 +9,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        gcc_v: [12] # Version of GFortran we want to use.
+        gcc_v: [15] # Version of GFortran we want to use.
         include:
         - os: ubuntu-latest
           os-arch: linux-x86_64