bazel: Bump to latest 4.x release: 4.2.4

Includes several fixes since 4.0, but not the breaking changes
introduced by 5.0.

Change-Id: Ibb8c134b164cba4275eded39a7c3c31889fedd89

.bazelversion

diff --git a/.bazelversion b/.bazelversion
index fcdb2e1..cf78d5b 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-4.0.0
+4.2.4