Jenkinsfile: use Ubuntu 20.04 for DCO stage

Also switching to use arm64, as all amd64 stages have moved to GitHub actions,
so using arm64 allows the same machine to be used for tests after the DCO check
completed.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Jenkinsfile

@@ -39,7 +39,7 @@ pipeline {
                 beforeAgent true
                 expression { params.dco }
             }
-            agent { label 'amd64 && ubuntu-1804 && overlay2' }
+            agent { label 'arm64 && ubuntu-2004' }
             steps {
                 sh '''
                 docker run --rm \