Bump Bazel version to 3.5.1

This latest release includes cherry picks, a new feature and some
important changes [1].

[1] https://github.com/bazelbuild/bazel/releases/tag/3.5.1

Change-Id: I33c3c6bd76a05e1700788a202d1c80955834f4c4

.bazelversion

diff --git a/.bazelversion b/.bazelversion
index 1545d96..d5c0c99 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-3.5.0
+3.5.1