Follow-up to #4194 (v3 execution app + core-helper removal). The v3
(engine V1) is end-of-lifed and enforced off in prod, so this removes a
self-contained slice of the remaining dead v3 code while **keeping every
user-facing deprecation message** - a user still on v3 must still be
told to upgrade.
## Legacy dev websocket
`app/v3/handleWebsockets.server.ts` backs the `/ws` transport used
**only** by the legacy v3 `trigger dev` CLI (v4 dev uses a different
transport). It's now authenticate-then-close with
`V3_DEV_DEPRECATION_MESSAGE`, so an old CLI is still told what to do -
only the legacy `AuthenticatedSocketConnection` / `DevQueueConsumer`
execution behind it (which can no longer run) is removed.
- Deleted `app/v3/authenticatedSocketConnection.server.ts` (its only
consumer).
- `engineDeprecation.server.ts` and the deprecation message constants
are untouched.
## Docs
Deleted the intentionally-legacy "Docker (legacy)" self-hosting page
(`open-source-self-hosting.mdx`) and redirected
`/open-source-self-hosting` (+ the existing
`/v3/open-source-self-hosting` alias) to `/self-hosting/overview`;
repointed the two inbound links. The current `self-hosting/*` docs
already describe the v4 (single supervisor) setup.
## Deliberately out of scope
Despite the branch name, this PR does **not** touch MarQS or the
socket.io coordinator/provider namespaces. Investigation found MarQS is
entangled with **live v2** queue/metrics/concurrency/project-cleanup
code (`runQueue`, `queueSizeLimits`, `taskRunConcurrencyTracker`,
`EnvironmentQueuePresenter`, `registerProjectMetrics`, `deleteProject`),
so it needs a per-file reviewed pass, not a bulk delete. That remainder
stays on TRI-11883.
refs TRI-11883
nicktrn
0631c8373c