Removing deprecated Ubuntu 20.04 runner (#11)

.github/workflows/continuous-integration.yml

@@ -32,14 +32,6 @@ jobs:
             compiler: 'clang-15'
             ci-args: '--linter clang-tidy'
             ci-tag: '-clang-tidy'
-          - runner: 'ubuntu-20.04'
-            compiler: 'gcc-9'
-            ci-args: ''
-            ci-tag: ''
-          - runner: 'ubuntu-20.04'
-            compiler: 'clang-10'
-            ci-args: ''
-            ci-tag: ''
           - runner: 'ubuntu-22.04'
             compiler: 'clang-13'
             ci-args: ''