Upgrade to Spring Framework 6.1.21

Closes gh-1077

Author: rainboyan

gradle/libs.versions.toml

@@ -63,7 +63,7 @@ snakeyaml = "2.2"
 spock = "2.3-groovy-4.0"
 spring-boot = "3.3.13"
 spring-dependency-management = "1.1.7"
-spring-framework = "6.1.20"
+spring-framework = "6.1.21"
 tomcat = "10.1.41"
 xhtmlrenderer = "R8"