Discourage customers from sending events to GitLab's Snowplow collector

What does this MR do?

In the Snowplow Guide, we describe how to configure a GitLab instance to send events to a collector. We reveal the address of our collector, which could lead to self-managed instances sending us their events.

Since we do not want self-managed events to be sent to our collector, this section is changed to show a more generic example.

For now, the address of our collector remains accessible in the handbook: https://about.gitlab.com/handbook/business-technology/data-team/platform/snowplow/. If we see an increasing number of self-managed events, we could remove the collector address from there as well.

Related to #322155 (closed)

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