Fixes https://github.com/coder/internal/issues/272
* Increases healthcheck timeout in tests. This seems to be the most
usual cause of test failures.
* Adds a non-nilness check before caching a healthcheck report.
* Modifies the HTTP response code to 503 (was 404) when no healthcheck
report is available. 503 seems to be a better indicator of the server
state in this case, whereas 404 could be misinterpreted as a typo in the
healthcheck URL.
Currently, importing `codersdk` just to interact with the API requires
importing tailscale, which causes builds to fail unless manually using
our fork.
Moves healthcheck report-related structs from coderd/healthcheck to codersdk
This prevents an import cycle when adding a codersdk.Client method to hit /api/v2/debug/health.
* 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
