Remove reference to 5.13 RC1

Author: sbrannen

README.md

@@ -13,7 +13,6 @@ This repository is the home of JUnit Platform, Jupiter, and Vintage.
 ## Latest Releases
 
 - General Availability (GA): [JUnit 5.13.2](https://github.com/junit-team/junit-framework/releases/tag/r5.13.2) (June 24, 2025)
-- Preview (Milestone/Release Candidate): [JUnit 5.13.0-RC1](https://github.com/junit-team/junit-framework/releases/tag/r5.13.0-RC1) (May 16, 2025)
 
 ## Documentation