ci: remove ubuntu 18 from test matrix (#8)

Author: wpbonelli

.github/workflows/test.yml

@@ -19,7 +19,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-22.04, ubuntu-20.04, ubuntu-18.04, macos-12, macos-11, windows-2022, windows-2019]
+        os: [ubuntu-22.04, ubuntu-20.04, macos-12, macos-11, windows-2022, windows-2019]
         compiler: [gcc]
         version: [12, 11, 10, 9, 8, 7, 6, 5]
     steps: