cmd/tsconnect: update to xterm.js 5.1

It includes xtermjs/xterm.js#4216, which improves handling of some
escape sequences. Unfortunately it's not enough to fix the issue
with `ponysay`, but it does not hurt to be up to date.

Updates #6090

Signed-off-by: Mihai Parparita <mihai@tailscale.com>
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