Remove Ubuntu 18.04 from linux-builds

Author: githubuser0xFFFF

.github/workflows/linux-builds.yml

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