[CALCITE-4940] Upgrade log4j2 version to 2.16.0

The 2.16.0 release has two additional fixes related to CVE-2021-4422 for
hardening security.
diff --git a/gradle.properties b/gradle.properties
index 3fac654..52b6765 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -72,7 +72,7 @@
 jetty.version=9.4.44.v20210927
 junit.version=4.12
 kerby.version=1.1.1
-log4j2.version=2.15.0
+log4j2.version=2.16.0
 mockito.version=2.23.4
 protobuf.version=3.17.1
 scott-data-hsqldb.version=0.1