cmd/tsshd: switch from github.com/kr/pty to github.com/creack/pty

The kr/pty module moved to creack/pty per the kr/pty README[1].

creack/pty brings in support for a number of OS/arch combos that
are lacking in kr/pty.

Run `go mod tidy` while here.

[1] https://github.com/kr/pty/blob/master/README.md

Signed-off-by: Aaron Bieber <aaron@bolddaemon.com>
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