CI: Remove ubuntu-20.04 runner as it's no longer available

Author: BlackMark29A

.github/workflows/libenvpp_ci.yml

@@ -8,7 +8,7 @@ jobs:
       matrix:
         build-type: [Debug, Release]
         compiler: [g++, clang++]
-        os: [ubuntu-latest, ubuntu-20.04, macos-latest]
+        os: [ubuntu-latest, macos-latest]
 
     runs-on: ${{ matrix.os }}