Update to GAE 1.9.78

Fixes gh-7893

gradle.properties

@@ -1,5 +1,5 @@
 aspectjVersion=1.9.3
-gaeVersion=1.9.76
+gaeVersion=1.9.78
 springBootVersion=2.2.4.RELEASE
 version=5.3.0.BUILD-SNAPSHOT
 org.gradle.jvmargs=-Xmx3g -XX:MaxPermSize=2048m -XX:+HeapDumpOnOutOfMemoryError