client: Send telemetry events with `Content-Type: application/json` (#18886)

This PR updates the telemetry events sent to collab to use
`Content-Type: application/json` instead of `Content-Type: text/plain`.

The POST bodies are JSON, so `application/json` is the correct MIME
type.

I suspect the `text/plain` is a remnant from when the events were still
going through Vercel.

Release Notes:

- N/A