Remove 'ubuntu-18.04' from os list

Author: davidjarzebowski

.github/workflows/main.yml

@@ -10,7 +10,7 @@ jobs:
         # macos-latest references are kept here for historical purposes. removed macos-latest from the 
         #matrix as it is not a typical case for users and causes a lot of friction with other linux-based 
         # installs. Recommend developing on codespaces or using an ubuntu container. 
-        os: ['ubuntu-22.04', 'ubuntu-20.04', 'ubuntu-18.04']
+        os: ['ubuntu-22.04', 'ubuntu-20.04']
       fail-fast: false
     runs-on: ${{ matrix.os }}
     steps: