Fixes#8218
Removes `wsconncache` and related "is legacy?" functions and API calls that were used by it.
The only leftover is that Agents still use the legacy IP, so that back level clients or workspace proxies can dial them correctly.
We should eventually remove this: #11819
fixes#10531
Adds a check for `version` on connection to the Agent API websocket endpoint. This is primarily for future-proofing, so that up-level agents get a sensible error if they connect to a back-level Coderd.
It also refactors the location of the `CurrentVersion` variables, to be part of the `proto` packages, since the versions refer to the APIs defined therein.
This one is huge, and I'm sorry.
The problem is that once I change `tailnet.Conn` to start doing v2 behavior, I kind of have to change it everywhere, including in CoderSDK (CLI), the agent, wsproxy, and ServerTailnet.
There is still a bit more cleanup to do, and I need to add code so that when we lose connection to the Coordinator, we mark all peers as LOST, but that will be in a separate PR since this is big enough!
wsproxy also needs to be updated to use tailnet v2 because the `tailnet.Conn` stores peers by ID, and the peerID was not being carried by the JSON protocol. This adds a query param to the endpoint to conditionally switch to the new protocol.
* chore: add /v2 to import module path
go mod requires semantic versioning with versions greater than 1.x
This was a mechanical update by running:
```
go install github.com/marwan-at-work/mod/cmd/mod@latest
mod upgrade
```
Migrate generated files to import /v2
* Fix gen
