Update to Spring Boot 2.2.6.RELEASE

Fixes gh-8252

gradle.properties

@@ -1,5 +1,5 @@
 aspectjVersion=1.9.5
 gaeVersion=1.9.79
-springBootVersion=2.2.4.RELEASE
+springBootVersion=2.2.6.RELEASE
 version=5.2.3.BUILD-SNAPSHOT
 org.gradle.jvmargs=-Xmx3g -XX:MaxPermSize=2048m -XX:+HeapDumpOnOutOfMemoryError