opal-ci: Remove ubuntu-18.04

Standard support for Ubuntu 18.04 ended on May 31, 2023.

Signed-off-by: Reza Arbab <[email protected]>

Author: rarbab

.github/workflows/docker-builds-checks.yml

@@ -12,7 +12,7 @@ jobs:
       fail-fast: false
 
       matrix:
-        os: [ ubuntu-18.04, ubuntu-20.04, ubuntu-22.04, ubuntu-rolling, fedora40, fedora41, fedora-rawhide, docs ]
+        os: [ ubuntu-20.04, ubuntu-22.04, ubuntu-rolling, fedora40, fedora41, fedora-rawhide, docs ]
 
     steps:
       - uses: actions/checkout@v4