Updated gen_release_jobs() method to only test on ubuntu-20.04

This patch will be removed after testing and before review

Author: minosgalanakis

vars/gen_jobs.groovy

@@ -576,16 +576,7 @@ def gen_release_jobs(label_prefix='', run_examples=true) {
     }
 
     if (env.RUN_ALL_SH == "true") {
-        for (component in common.available_all_sh_components['ubuntu-16.04']) {
-            jobs = jobs + gen_all_sh_jobs('ubuntu-16.04', component, label_prefix)
-        }
-        for (component in (common.available_all_sh_components['ubuntu-18.04'] -
-                           common.available_all_sh_components['ubuntu-16.04'])) {
-            jobs = jobs + gen_all_sh_jobs('ubuntu-18.04', component, label_prefix)
-        }
-        for (component in (common.available_all_sh_components['ubuntu-20.04'] -
-                           common.available_all_sh_components['ubuntu-18.04'] -
-                           common.available_all_sh_components['ubuntu-16.04'])) {
+        for (component in (common.available_all_sh_components['ubuntu-20.04'])) {
             jobs = jobs + gen_all_sh_jobs('ubuntu-20.04', component, label_prefix)
         }
     }