Start building against Spring Framework 5.2.15 snapshots

See gh-26425

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

@@ -1673,7 +1673,7 @@ bom {
 			]
 		}
 	}
-	library("Spring Framework", "5.2.14.RELEASE") {
+	library("Spring Framework", "5.2.15.BUILD-SNAPSHOT") {
 		group("org.springframework") {
 			imports = [
 				"spring-framework-bom"