add reference to Spring Boot 3

Author: rieckpil

README.md

@@ -125,6 +125,8 @@ Even though the technical setup for your day-to-day projects might differ, the t
 
 `code-along` branch: [![Build & Test Maven Project (code-along)](https://github.com/rieckpil/testing-spring-boot-applications-masterclass/workflows/Build%20&%20Test%20Maven%20Project/badge.svg?branch=code-along)](https://github.com/rieckpil/testing-spring-boot-applications-masterclass/actions/workflows/maven.yml?query=branch%3Acode-along)
 
+`spring-boot-3` branch: Keeping the Spring Boot 3 version alive for those who are still on Spring Boot 3 and want to learn testing with that version.
+
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