Automate the per-release go.mod k8s version bump (gen-compdocs, genref)

[Caesarsage]

The Kubernetes staging module pins in gen-compdocs/go.mod (in the replace block) and genref/go.mod (direct requires) are hand-edited every release. Tedious and error-prone. Dependabot only covers npm/bundler and can't do this anyway: the staging set must move together in one coordinated bump, not one PR per module.

Proposed changes

• Move set_version_dirs.sh → hack/ (alongside gen-enum-swagger.sh, verify-enum-swagger.sh); update the createversiondirs Makefile path.
• Add hack/update-go-mod.sh + make update-mod: given K8S_RELEASE, bump the coordinated k8s set in gen-compdocs and genref, then go mod tidy.

export K8S_RELEASE=1.37.0
make update-mod

What it bumps: every k8s.io/* pinned to a concrete v0.<minor>.<patch> (in require or replace) → v0.<newminor>.0, plus k8s.io/kubernetes (where required) → v1.<newminor>.0. What it leaves alone: pseudo-versions (gengo, kube-openapi, utils) and independently-versioned modules (klog/v2, system-validators).

Depends on

#462: k8s.io/kubernetes must be a versioned require (not the local-path replace) for the main-module bump to apply.

Test (locally)

• Selection — bumped to 1.35 and diffed: every staging replace/require and k8s.io/kubernetes moved; klog/gengo/kube-openapi/utils untouched.
• End-to-end — run at 1.36 (current): both repos processed, go mod tidy clean, idempotent no-op.