PB-1068 refactor 1: Replace addRepositoryHostToSSHKnownHosts with StrictHostKeyChecking=accept-new

[zhming0]

Description

Simplifies SSH host key verification by using OpenSSH's StrictHostKeyChecking=accept-new option instead of running ssh-keyscan before git operations.

• Set GIT_SSH_COMMAND="ssh -o StrictHostKeyChecking=accept-new" when SSHKeyscan is enabled
• Removed ssh-keyscan invocations and known_hosts file manipulation
• Deleted ssh.go, knownhosts.go and their tests (~450 lines removed)

The new approach is important reducing our reliance on Git binary, making it slightly easier for my subsequent git abstraction PR.

• Eliminates dependency on ssh-keyscan binary
• Removes Windows-specific path hackery to find SSH tools bundled with Git
• Simpler, fewer moving parts

One important note is this PR raises our minimum SSH version requirement to ~2017. You can vote yes or no 🙂 .

Context

part of PB-1068

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go tool gofumpt -extra -w .)

Disclosures / Credits

LLM powered this time