Update to Spring Framework 5.3.20

Closes gh-750

gradle.properties

@@ -2,7 +2,7 @@ version=0.3.0-SNAPSHOT
 org.gradle.jvmargs=-Xmx3g -XX:MaxPermSize=2048m -XX:+HeapDumpOnOutOfMemoryError
 org.gradle.parallel=true
 org.gradle.caching=true
-springFrameworkVersion=5.3.19
+springFrameworkVersion=5.3.20
 springSecurityVersion=5.6.3
 springJavaformatVersion=0.0.31
 springJavaformatExcludePackages=org/springframework/security/config org/springframework/security/oauth2