Planned v15 EOL => 2026-12-15

As per our [Version Support](https://github.com/graphql/graphql-js/#version-support) and [End-of-Life](https://github.com/graphql/graphql-js/#end-of-life-eol-schedule) policies, the release of v17 also gives us an opportunity to wish a warm farewell to v15.

Per policy, we are announcing that on 2026-12-15 (six months after the v17 release date), we will stop providing updates for v15, including updates for critical security issues. At that point, we also plan to deprecate the v15 release line on `npm`.