[Security] Bump sshpk from 1.13.1 to 1.15.1
[Security] Bump sshpk from 1.13.1 to 1.15.1

Bumps [sshpk](https://github.com/joyent/node-sshpk) from 1.13.1 to 1.15.1. **This update includes security fixes.**
- [Release notes](https://github.com/joyent/node-sshpk/releases)
- [Commits](https://github.com/joyent/node-sshpk/compare/v1.13.1...v1.15.1)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
inspireteam/plunger,inspireteam/plunger