Publish the Mongoid 8.x end-of-life notice in January 2027, as proposed in RUBY-3961 (wording approved in #ruby-team, Oct 7, 2026).

Notice text

Mongoid 8.x / 8.1 — January 2027 notice

End-of-Life Notice
The Mongoid 8.x series will receive maintenance and security updates through July 31, 2027. After that date, it will no longer be supported or receive maintenance or security updates. If you are using Mongoid 8.x, please plan to upgrade to a supported Mongoid version before then.

Context

* 8.x is roughly 35% of Mongoid customers; a January notice gives six full months of support runway (through July 31, 2027).
* Target date: end of January 2027.
* Part of agreeing to deprecate everything up to 9.0 with messaging in Q1 2027, per the [Process to drop support|https://docs.google.com/document/d/1yt8kIw5098WGeUgYajRbCFCEkUsdhM33-P3fwVH_ehE/edit].
* Discussion: [Slack thread, Oct 7, 2026|https://mongodb.enterprise.slack.com/archives/GH2HEDYBF/p1791400982588299]

Acceptance criteria

* The notice is published in the agreed channels (docs/release notes/etc.).
* The EOL date (July 31, 2027) matches the notice text above.