Adds a reusable Claude Code skill (`.claude/skills/retiring-server-versions/SKILL.md`) to the specifications repo, documenting a repeatable process for cleaning up dead version-gated tests, prose, and pseudocode after the minimum supported MongoDB server version is raised.

This was distilled from the DRIVERS-3439 pre-4.2 cleanup, which took many review rounds to find all the ways dead version references hide in this repo (vacuous "at-floor" qualifiers, wire-version-only phrasing, stale illustrative examples, unrunnable YAML tests gated by maxServerVersion, stale generated-file naming, etc.). The skill captures those lessons – including tested, verified grep/find/awk commands for each pattern – so the next retirement (e.g. eventually retiring MongoDB 4.2 itself) is systematic rather than starting from scratch.

The skill was validated by running it (in dry-run/discovery mode only, no edits made) against a hypothetical 4.2-to-4.4 retirement in this repo, and iterated on based on that trial run plus two rounds of independent critical review.