Start building against Spring Framework 5.3.1 snapshots

See gh-31319

spring-boot-project/spring-boot-dependencies/build.gradle

@@ -1687,7 +1687,7 @@ bom {
 			]
 		}
 	}
-	library("Spring Framework", "5.3.20") {
+	library("Spring Framework", "5.3.21-SNAPSHOT") {
 		prohibit("[6.0.0-M1,)") {
 			because "we upgrade in Spring Boot 3.x"
 		}