capver, types: bump to tailscale v1.98, drop LegacyDERPString

Regenerate capver for tailscale v1.98 — `MinSupportedCapabilityVersion`
slides 109 → 113 (v1.78 → v1.80). v1.80+ clients understand
`Node.HomeDERP`, which superseded `LegacyDERPString` upstream at capver
111. Drops the emit, plumbing, the trip-wire that caught this cleanup,
the golden test fixtures, and the integration check on
`peer.LegacyDERPString()` — the sibling `HomeDERP` check covers intent.

`v1.98` was added by hand to `capver_generated.go` because tailscale
skipped publishing v1.97/v1.98 to ghcr until late in the cycle; a clean
regeneration produces the same content.