remove Ubuntu 18.04 as it is EOL since June 2023

Author: christianrauch

.github/workflows/ci.yaml

@@ -14,11 +14,6 @@ jobs:
         matrix:
           os: [ubuntu-latest, macos-latest]
           python: [3.7, 3.8, 3.9]
-          include:
-          - os: ubuntu-18.04
-            python: 2.7
-          - os: ubuntu-18.04
-            python: 3.6
       name: bloom tests
       runs-on: ${{matrix.os}}