Bump bazel version

With the new bazel version FloggerLogString errors such as fixed by
change I5cc1f2eae are caught by the build now.

Release-Notes: skip
Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I6550cca076649bedbcb0250118941b3225fee541

.bazelversion

diff --git a/.bazelversion b/.bazelversion
index 0062ac9..c7cb131 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-5.0.0
+5.3.1