stop using the default, global HTTP client to avoid `CloseIdleConnections` flakes

Some places in coder/coder, [like this](https://github.com/coder/coder/blob/29da25c3964dfb3f1c934e368be8ce65ac302866/coderd/telemetry/telemetry.go#L353), use the default, global HTTP client:

```go
resp, err := http.DefaultClient.Do(req)
if err != nil {
return xerrors.Errorf("perform request: %w", err)
}
```

This causes flakes in our tests with the message `net/http: HTTP/1.x transport connection broken: http: CloseIdleConnections called`:

```
error= perform request:
github.com/coder/coder/v2/coderd/telemetry.(*remoteReporter).deployment
/Users/runner/work/coder/coder/coderd/telemetry/telemetry.go:355
- Post "http://127.0.0.1:56934/deployment": net/http: HTTP/1.x transport connection broken: http: CloseIdleConnections called
```

I think the flake manifests when a test running in parallel in the same package calls `CloseIdleConnections` on the default client.

To avoid the flakes, we should refactor our code to stop using the global HTTP client and use local instances instead.

Related issues:

- https://github.com/coder/internal/issues/645
- https://github.com/coder/internal/issues/920
- https://github.com/coder/internal/issues/1016
- https://github.com/coder/internal/issues/835
- https://github.com/coder/internal/issues/582