refactor(platform): factor the cozystack-version stamp into a shared migration helper #2781

Migration 43 (#2602) had to inline-copy migration 42's labeled ConfigMap heredoc to avoid stripping the platform.cozystack.io/no-delete label on a label-less apply by the same field manager. Every future migration that stamps the version must remember to carry the no-delete label by hand — the bug class isn't eliminated, just patched for 43.

Extract the version stamp into a shared helper (sourced by each migrations/<N> script) that always emits the labeled manifest, so migration 44+ cannot drift back to a label-less apply.

Refs: #2602 (review by Aleksei Sviridkin (@lexfrei)), packages/core/platform/images/migrations/migrations/{42,43}