Bump sshkit from 1.21.5 to 1.21.6

Bumps [sshkit](https://github.com/capistrano/sshkit) from 1.21.5 to 1.21.6.
- [Release notes](https://github.com/capistrano/sshkit/releases)
- [Commits](capistrano/sshkit@v1.21.5...v1.21.6)

---
updated-dependencies:
- dependency-name: sshkit
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -277,7 +277,7 @@ GEM
       simplecov_json_formatter (~> 0.1)
     simplecov-html (0.12.3)
     simplecov_json_formatter (0.1.4)
-    sshkit (1.21.5)
+    sshkit (1.21.6)
       net-scp (>= 1.1.2)
       net-ssh (>= 2.8.0)
     standard (1.32.0)