Update ubuntu18 to ubuntu20 (#611)

[ROCm/rccl commit: d700a94918]

projects/rccl/.jenkins/staticanalysis.groovy

@@ -63,6 +63,6 @@ ci: {
 
     properties(auxiliary.addCommonProperties([pipelineTriggers([cron('0 1 * * 6')])]))
     stage(urlJobName) {
-        runCI([ubuntu18:['cpu']], urlJobName)
+        runCI([ubuntu20:['cpu']], urlJobName)
     }
 }