Match Bazel version from core

stable-3.1 uses Bazel version 4.0.0.

Change-Id: I08280dd7a3a1caa8c6f2b0ed7af98069785fabc9
diff --git a/.bazelversion b/.bazelversion
index 7c69a55..fcdb2e1 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-3.7.0
+4.0.0