Rename UsagePingController to ServicePingController and refactor/fix specs

What does this MR do?

Part of #332563 (closed). As part of the &5990 (closed), we want to rename Usage Ping to Service Ping.

Update: This MR now contains only backend changes, due to possible split deployment issues: https://docs.gitlab.com/ee/development/multi_version_compatibility.html#when-modifying-javascript

Follow-up frontend MR: !64449 (merged)

Does this MR meet the acceptance criteria?

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Availability and Testing

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