remove deprecated Ubuntu 18.04 from build matrix (#316)

Author: jmckenna

.github/workflows/build-linux.yml

@@ -6,7 +6,7 @@ jobs:
     build-matrix:
       strategy:
         matrix:
-          os: [ ubuntu-18.04, ubuntu-20.04 ]
+          os: [ ubuntu-20.04 ]
           option: [ minimal, default, maximal ]
       runs-on: ${{ matrix.os }}
       steps: