update CI for gfortran 10

Author: jvdp1

.github/workflows/CI.yml

@@ -73,14 +73,14 @@ jobs:
       working-directory: build
 
     - name: Test in-tree builds
-      if: contains( matrix.gcc_v, '9') # Only test one compiler on each platform
+      if: contains( matrix.gcc_v, '10') # Only test one compiler on each platform
       run: |
           cmake -DCMAKE_MAXIMUM_RANK=4 .
           cmake --build .
           cmake --build . --target test
 
     - name: Test manual makefiles
-      if: contains(matrix.os, 'ubuntu') && contains(matrix.gcc_v, '9')
+      if: contains(matrix.os, 'ubuntu') && contains(matrix.gcc_v, '10')
       run: |
         make -f Makefile.manual FYPPFLAGS="-DMAXRANK=4"
         make -f Makefile.manual test