security/ruby-sshkit: update to 1.21.4

1.21.4 (2023-02-20)

This is a gem housekeeping release. No user-facing changes.

Housekeeping

* Add functional test with vagrant on CI (#509) @colorbox
* Migrate CI build from Travis to GitHub Actions (#507) @colorbox
* Fix test by require 'set' (#508) @colorbox
* Bump toolmantim/release-drafter from 5.20.1 to 5.22.0 (#501, #502, #506)
  @dependabot

(taca)