Remove ubuntu-18.04

https://github.blog/changelog/2022-08-09-github-actions-the-ubuntu-18-04-actions-runner-image-is-being-deprecated-and-will-be-removed-by-12-1-22/

Author: dentarg

.github/workflows/test.yml

@@ -14,7 +14,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-18.04, ubuntu-20.04, ubuntu-22.04 ]
+        os: [ ubuntu-20.04, ubuntu-22.04 ]
 
     name: ${{ matrix.os }}
     runs-on: ${{ matrix.os }}