update to gradle 6.9, jdk 15, update findbugs
diff --git a/lib/java/gradle/wrapper/gradle-wrapper.properties b/lib/java/gradle/wrapper/gradle-wrapper.properties
index 5028f28..ec991f9 100644
--- a/lib/java/gradle/wrapper/gradle-wrapper.properties
+++ b/lib/java/gradle/wrapper/gradle-wrapper.properties
@@ -1,5 +1,5 @@
distributionBase=GRADLE_USER_HOME
distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-5.6.4-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.9.2-bin.zip
zipStoreBase=GRADLE_USER_HOME
zipStorePath=wrapper/dists