lang/ruby30: Update pkg-message

In Ruby 3.0 following standard libraries are distributed as bundled
gems.

- minitest
- power_assert
- test-unit
- typeprof
- rexml
- rss

But this port doesn't include bundled gems in its package. So add
message to pkg-message that they are provided as separate ports.

PR:		259757
Approved by:	maintainer timeout (> 3 weeks)