Remove Ubuntu 20.04 from CI as it's no longer supported

Author: olszomal

.github/workflows/ci.yml

@@ -24,11 +24,6 @@ jobs:
             compiler: gcc
             os: ubuntu-22.04
             generator: Unix Makefiles
-          - id: ubuntu-20.04
-            triplet: x64-linux
-            compiler: gcc
-            os: ubuntu-20.04
-            generator: Unix Makefiles
           - id: macOS-3
             openssl: 'openssl@3'
             triplet: x64-osx