Update Ruby version for building documentation

[zimeg]

Summary

This PR updates the Ruby version used locally when building documentation to match what GitHub Pages uses.

Category

• [ ] bolt (Bolt for Java)
• [ ] bolt-{sub modules} (Bolt for Java - optional modules)
• [ ] slack-api-client (Slack API Clients)
• [ ] slack-api-model (Slack API Data Models)
• [ ] *slack-api--kotlin-extension** (Kotlin Extensions for Slack API Clients)
• [ ] slack-app-backend (The primitive layer of Bolt for Java)
• [x] docs (Documentation)

Requirements

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (6a285c3) 74.32% compared to head (ddc091f) 74.34%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1261 +/- ## ============================================ + Coverage 74.32% 74.34% +0.02% Complexity 4121 4121 ============================================ Files 443 443 Lines 13078 13078 Branches 1322 1322 ============================================ + Hits 9720 9723 +3 + Misses 2586 2582 -4 - Partials 772 773 +1 ```

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[zimeg]

@filmaj haha exact same thought... But oh well I guess. Thanks for the review!