Update to Spring Boot 2.4.0-M1

Closes gh-8787

gradle.properties

@@ -1,6 +1,6 @@
 aspectjVersion=1.9.3
 gaeVersion=1.9.80
-springBootVersion=2.3.0.RELEASE
+springBootVersion=2.4.0-M1
 version=5.4.0-SNAPSHOT
 kotlinVersion=1.3.72
 org.gradle.jvmargs=-Xmx3g -XX:MaxPermSize=2048m -XX:+HeapDumpOnOutOfMemoryError