[Security] Bump sshpk from 1.13.0 to 1.15.2

Bumps [sshpk](https://github.com/joyent/node-sshpk) from 1.13.0 to 1.15.2. **This update includes security fixes.**
- [Release notes](https://github.com/joyent/node-sshpk/releases)
- [Commits](TritonDataCenter/node-sshpk@v1.13.0...v1.15.2)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Author: dependabot-support