Noting jenkinsci/workflow-cps-plugin#543 in the upgrade guide (#5371)

Author: basil

content/_data/upgrades/2-346-3.adoc

@@ -1 +1,6 @@
-No notable changes requiring upgrade notes.
+==== OpenJDK upgraded to 11.0.16
+
+The version of OpenJDK in the official Docker image has been upgraded from 11.0.15 to 11.0.16.
+This exposes a metaspace memory leak in Pipeline: Groovy 2692.v76b_089ccd026 and earlier.
+After upgrading Jenkins to 2.346.3, upgrade Pipeline: Groovy to 2705.v0449852ee36f or later.
+It is generally recommended to upgrade all plugins after upgrading Jenkins core.