iox-#2270 Update release notes

Author: elBoberido

doc/website/release-notes/iceoryx-unreleased.md

@@ -223,6 +223,7 @@
 - Automate check for test cases to have UUIDs [\#1540](https://github.com/eclipse-iceoryx/iceoryx/issues/1540)
 - Add Thread Sanitizer to build and test workflow [\#692](https://github.com/eclipse-iceoryx/iceoryx/issues/692)
 - Document workaround for documentation in internal headers [#1293](https://github.com/eclipse-iceoryx/iceoryx/issues/1293)
+- The default branch is now `main` [#2270](https://github.com/eclipse-iceoryx/iceoryx/issues/2270)
 
 **New API features:**