Update to Spring Framework 5.3.15

Closes gh-605

gradle/dependency-management.gradle

@@ -1,5 +1,5 @@
 if (!project.hasProperty("springVersion")) {
-	ext.springVersion = "5.3.13"
+	ext.springVersion = "5.3.15"
 }
 
 if (!project.hasProperty("springSecurityVersion")) {