Update 3rdparty README.md (Clipper2 1.5.4) (#2564)

Author: aismann

3rdparty/README.md

@@ -32,7 +32,7 @@
 
 ## Clipper2
 - [![Upstream](https://img.shields.io/github/v/tag/AngusJohnson/Clipper2?label=Upstream)](https://github.com/AngusJohnson/Clipper2)
-- Version: 1.5.3
+- Version: 1.5.4
 - License: BSL-1.0
 
 ## ConcurrentQueue