Start building against Spring Framework 6.0.21 snapshots

See gh-40825

Author: wilkinsona

gradle.properties

@@ -12,7 +12,7 @@ junitJupiterVersion=5.9.3
 kotlinVersion=1.8.22
 mavenVersion=3.9.4
 nativeBuildToolsVersion=0.9.28
-springFrameworkVersion=6.0.20
+springFrameworkVersion=6.0.21-SNAPSHOT
 tomcatVersion=10.1.24
 
 kotlin.stdlib.default.dependency=false