Downgrade plumed ubuntu runner

Author: danielhollas

.github/workflows/gfortran.yml

@@ -373,7 +373,7 @@ jobs:
 
   plumed_build:
     name: PLUMED build
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-22.04
     needs: basic_build
     strategy:
       fail-fast: false