CI: Remove ubuntu-20.04 runner (retired).

Author: rdzman

.github/workflows/continuous-integration.yml

@@ -14,7 +14,7 @@ jobs:
       matrix:
         platform: [octave]
 #        os: [macos-10.15, macos-11.0, macos-12.0, ubuntu-18.04, ubuntu-20.04, ubuntu-22.04]
-        os: [macos-latest, ubuntu-20.04, ubuntu-22.04, ubuntu-24.04, ubuntu-latest]
+        os: [macos-latest, ubuntu-22.04, ubuntu-24.04, ubuntu-latest]
         include:
         - platform: matlab
           os: ubuntu-latest