ipn/ipnstate: add PeerStatus.KeyExpiry for tailscale status --json

Fixes #6712

Change-Id: I817cd5342fac8a956fcefda2d63158fa488f3395
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f