Update release notes

Author: mpkorstanje

documentation/modules/ROOT/partials/release-notes/release-notes-6.1.0-M2.adoc

@@ -47,6 +47,8 @@ repository on GitHub.
 * https://www.junit-pioneer.org/[JUnit Pioneer]'s `DefaultLocaleExtension` and
   `DefaultTimeZoneExtension` are now part of the JUnit Jupiter. Find examples in the
   xref:writing-tests/built-in-extensions.adoc#DefaultLocaleAndTimeZone[User Guide].
+* The `UniqueId.uniqueIdFormat` field has been removed, reducing the size of `UniqueId`
+  objects.  
 
 [[v6.1.0-M2-junit-vintage]]
 === JUnit Vintage