[v0.21.x-branch] Backport #10813: Drop tor v2 onion production, keep wire codec faithful

[github-actions]

Backport of #10813

---

Context

This PR is split out from #10795 ("multi: remove deprecated RPCs and
config flags scheduled for 0.21"), which removes everything that was
announced for removal in 0.21 via the 0.20 release notes. That parent
PR was carved into smaller, independently-reviewable PRs by topic;
this is the tor v2 piece.

Tor v2 onion services have been obsolete since October 2021, when
the Tor network dropped support for them. The --tor.v2 flag was deprecated
in 0.20 with a scheduled removal in 0.21, and that's what this PR
ships.

lnd should not produce v2 addresses on any code path anymore, but it
must still verify signatures on and re-broadcast peer
NodeAnnouncement messages that carry v2 entries alongside still-
valid addresses, so we cannot strip v2 from the wire codec.

This PR draws that line: stop producing v2 everywhere, keep the codec
byte-faithful.

What changes

Stop producing v2

• lncfg.ParseAddressString rejects v2 .onion strings at the
  operator-input boundary, so --externalip, --listen,
  lncli connect <pubkey>@<v2>.onion, and
  lncli wtclient towers add <pubkey>@<v2>.onion fail fast with a
  clear error.
• The --tor.v2 flag and its sample-config entry are gone (the
  deprecation announced in 0.20).
• The self-announcement builder filters any v2 entry inherited from a
  previously stored self-node, so upgraded lnd instances never re-emit
  their own v2 onions.
• tor.OnionHostToFakeIP (the OnionCat v2 → fake-IPv6 helper) and the
  matching decoder cluster (FakeIPToOnionHost, IsOnionFakeIP,
  onionPrefixBytes) are removed. The scheme is v2-only by
  construction — v3 decodes to 35 bytes and doesn't fit in an IPv6
  address.
• On-disk legacy v2 keys (RSA1024:…) are rejected with the new
  ErrNonV3OnionKey rather than silently regenerating a fresh v3
  service, which would change the advertised onion identity.
• tor.OnionType enum, AddOnionConfig.Type, and
  watchtower.Config.Type are removed — only V3 was left, the field
  was a no-op pass-through.

Keep the wire codec faithful

lnwire.WriteOnionAddr, graph/db.encodeOnionAddr, and the matching
decoders round-trip v2 bytes so:

• DataToSign reproduces the exact bytes the remote peer signed,
• signature validation succeeds,
• the announcement is persisted to the graph DB and re-broadcast
  across restarts byte-for-byte.

RPC surfaces (GetNodeInfo, DescribeGraph) continue to expose the
full address set so external tools can independently reproduce and
verify the signed bytes.

Breaking changes / operator-facing notes

This is a breaking change for any node still configured with v2. The
--tor.v2 flag has been deprecated since 0.20 and v3 has been the
only working option network-side since October 2021, so there should
be no production node still relying on it, but the migration steps
are:

• A v2 .onion entry in lnd.conf (under externalip /
  tor.privatekeypath / wtclient.tower etc.) will now cause lnd to
  refuse to start. Remove the entry before upgrading.
• A pre-existing v2 private key file on disk (v2_onion_private_key,
  or a renamed key whose contents begin with RSA1024:) will cause
  the hidden-service setup to error out at startup with
  ErrNonV3OnionKey. Delete the file to let lnd generate a fresh v3
  service.
• Peer/tower addresses already persisted from a previous version
  still decode correctly through the wire-faithful codec, only new
  operator input is refused.
• GetNodeInfo / DescribeGraph continue to expose v2 entries when a
  remote peer's announcement contains them, so downstream tools that
  independently verify announcement signatures keep working.

[ziggie1984]

LGTM