Update to Spring Boot 2.2.4

Fixes gh-7909

gradle.properties

@@ -1,5 +1,5 @@
 aspectjVersion=1.9.5
 gaeVersion=1.9.76
-springBootVersion=2.2.0.RELEASE
+springBootVersion=2.2.4.RELEASE
 version=5.2.2.BUILD-SNAPSHOT
 org.gradle.jvmargs=-Xmx3g -XX:MaxPermSize=2048m -XX:+HeapDumpOnOutOfMemoryError