retrospectives: add yukik8 for 2026-08-05 ruby-tokyo - #2110

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: oss-gate/issue-cleaner@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/