Consider to cut new release for next preview release of Ruby

[hsbt]

👋 Hi, we have a plan to next preview release of Ruby after next week.

The master branch of ruby/ruby still points development version of rdoc. It's nice to move the release version like 6.15.1 or 6.16.0.

https://github.com/ruby/ruby/blob/master/gems/bundled_gems#L42

I can release that if you are busy. Please let me know anything.

[st0012]

Thanks for the heads up. I plan to cut a release after #1433 is merged.

[kou]

#1435 may be nice to have for the release.

[st0012]

Done: https://github.com/ruby/rdoc/releases/tag/v6.15.1