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
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!
