See #7568 for background on this.

We attempted to make Thread.to_java return java.lang.Thread by default in #7549 but decided that it was too risky to make such a breaking change. Instead, we will deprecate the behavior of to_java returning RubyThread by default in "9.5", and make the switch in "9.6".

Ideally all known uses of to_java => RubyThread will be fixed in preparation for the deprecation in #7568 so the eventual breakin API change will not affect users.