Rename tracker in Tracking in JH

Rename tracker in Tracking in JH.

Upstream related MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/91013

The current Gitlab::Tracking only supports Snowplow, and has some hard-coding.

JH wants to replace and extend the tracker supported in JH, so wants to rename the client to a more generic name:

Rename snowplow to tracker.