treewide: bump bazel to 8.3.1

This also bumps bazel_skylib, as it uses deprecated functionality.

Change-Id: I26d665967c14e5b3a437a3322f065ae1e2e63d9e
Reviewed-on: https://review.monogon.dev/c/monogon/+/4383
Tested-by: Jenkins CI
Reviewed-by: Leopold Schabel <leo@monogon.tech>
diff --git a/.bazelversion b/.bazelversion
index da15618..905c243 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-8.1.0
\ No newline at end of file
+8.3.1
\ No newline at end of file