Demote JavaUtilDate error prone bug pattern to warning severity

Upcoming Bazel release 5.0 includes recent error prone release that
flags JavaUtilDate bug pattern. Demote the severity to warning to not
block Bazel upgrade and promote it to error severity again, when all
places were adapted.

Bug: Issue 15492
Change-Id: I406ecbe4c460534a9d3aa50464c568d539dc9e80
diff --git a/tools/BUILD b/tools/BUILD
index 64044fc..48cc854 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -275,7 +275,7 @@
         "-Xep:JavaLocalTimeGetNano:ERROR",
         "-Xep:JavaPeriodGetDays:ERROR",
         "-Xep:JavaTimeDefaultTimeZone:ERROR",
-        "-Xep:JavaUtilDate:ERROR",
+        "-Xep:JavaUtilDate:WARN",
         # "-Xep:JdkObsolete:WARN",
         "-Xep:JodaConstructors:ERROR",
         "-Xep:JodaDateTimeConstants:ERROR",