Starting many (90+) SSH connections has caused us some issues such as
failed DNS lookups and hitting process file descriptor limits.
To mitigate this, patch SSHKit::Backend::Netssh to limit concurrency of
connection starts. We'll default to 30 at a time which seems to work
without issue, but can be configured via:
```
sshkit:
max_concurrent_starts: 10
```
Versions of net-ssh before 7.0 do not support the SHA-2 algorithm and result in mrsk not being able to connect to hosts using keys generated with it. net-ssh is also a dependency of sshkit, however, sshkit has a version requirement of >= 2.8.0 for net-ssh, so is not effective at ensuring mrsk has the version it needs to be the most compatible.
