Update release notes

Author: mpkorstanje

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

@@ -25,7 +25,8 @@ repository on GitHub.
 [[v6.1.0-M2-junit-platform-new-features-and-improvements]]
 ==== New Features and Improvements
 
-* ❓
+* The `UniqueId.uniqueIdFormat` field has been removed, reducing the size of `UniqueId`
+objects.
 
 [[v6.1.0-M2-junit-jupiter]]
 === JUnit Jupiter
@@ -48,6 +49,7 @@ repository on GitHub.
   `DefaultTimeZoneExtension` are now part of the JUnit Jupiter. Find examples in the
   xref:writing-tests/built-in-extensions.adoc#DefaultLocaleAndTimeZone[User Guide].
 
+
 [[v6.1.0-M2-junit-vintage]]
 === JUnit Vintage