Remove Ubuntu 18.04 from CI workflow

Author: xEnVrE

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
     strategy:
       matrix:
         build_type: [Release, Debug]
-        os: [ubuntu-18.04, ubuntu-20.04, windows-latest, macOS-latest]
+        os: [ubuntu-20.04, windows-latest, macOS-latest]
 
     steps:
     - uses: actions/checkout@master