README rewrite + release notes + major version tag

[rdhar]

Part of #574 (Phase 5) · tracking #568

• Rewrite README for the using: node action: remove external-tool prerequisites (gh, jq, md5sum, unzip, openssl, diff); document the minimum supported GHES version (Q2).
• Changelog / release notes summarising the migration; note zero input/output breaking changes.
• Cut a major version tag (e.g. v14); canary before moving the floating major alias from v13 → v14.