Update bazel-skylib in WORKSPACE setup (#2116)

**What type of PR is this?**

Dep update

**What package or component does this PR mostly affect?**

tests only

**What does this PR do? Why is it needed?**

Use the same version as in `MODULE.bazel`, which requires updating past
the split into `bazel_skylib` and `bazel_skylib_gazelle_plugin`.

**Which issues(s) does this PR fix?**

Unblocks
https://github.com/bazel-contrib/bazel-gazelle/pull/2100#discussion_r2135129237

**Other notes for review**