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.
Refactors the code that handles monitoring an agent websocket with pings and updating the connection times in the DB.
Consolidates v1 and v2 agent APIs under the same code for this.
One substantive change (not _just_ a refactor) is that I've made it so that we actually disconnect if the agent fails to respond to our pings, rather than the old behavior where we would update the database, but not actually tear down the websocket.
