Simplify DRA CI job node label selector

.ci/jobs.t/elastic+elasticsearch+periodic+dra-snapshots.yml

@@ -4,7 +4,7 @@
     workspace: /dev/shm/elastic+elasticsearch+%BRANCH%+periodic+dra-snapshot
     display-name: "elastic / elasticsearch # %BRANCH% - DRA snapshot"
     description: "Publishing Daily Releasable Artifacts (DRAs) of Elasticsearch %BRANCH% snapshots.\n"
-    node: "ubuntu-18.04 || ubuntu-20.04"
+    node: "ubuntu-20.04"
     builders:
       - inject:
           properties-file: '.ci/java-versions.properties'