Add service_ping category to Product Intelligence group

Why is this change being made?

In order to rename Usage Ping to Service Ping, as outlined in this proposal, one of the elements is to rename feature_category :usage_ping in gitlab-com to service_ping.

This requires us to add a new service_ping category for Product Intelligence group in data/stages.yml, and then define that category in data/categories.yml.

Author Checklist

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