Udpate release notes wrt "master"->"main" branch renaming

Author: cowtowncoder

release-notes/VERSION

@@ -9,6 +9,7 @@ Releases
 #122: RFC-4122 Obsoleted by RFC-9562 (document change)
  (pointed out by @akefirad)
 - Update to `oss-parent` v68 to switch to Central Portal publishing
+- Branch "master" renamed as "main"
 
 5.1.0 (02-Jun-2024)