github: Don't build on Ubuntu 18.04 anymore

The Ubuntu 18.04 image is deprecated and builds will start to fail
temporarily during four 24 hour periods from now until the final
deprecation on April 1st.  So better remove these runs now.

.github/workflows/linux.yml

@@ -135,7 +135,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ ubuntu-20.04, ubuntu-18.04 ]
+        os: [ ubuntu-20.04 ]
         test: [ all, nm ]
         compiler: [ gcc, clang ]
         exclude: