Remove Ubuntu:18.04 from OS tests

Ubuntu 18.04 EOL - https://ubuntu.com/about/release-cycle

Author: jakaskerl

.github/workflows/test-install-dependencies.yml

@@ -16,7 +16,7 @@
      runs-on: ubuntu-latest
      strategy:
        matrix:
-          container_image: ["fedora:34", "fedora:35", "fedora:36", "ubuntu:18.04", "ubuntu:20.04", "ubuntu:22.04", "ubuntu:rolling"]
+          container_image: ["fedora:34", "fedora:35", "fedora:36", "ubuntu:20.04", "ubuntu:22.04", "ubuntu:rolling"]
      container:
        image: ${{ matrix.container_image }}
      steps: