[chore] move event to registry

[trisch-me]

I have moved it to the registry as part of schema clean up but we can decide later if it should stay in this form based on outcome of #880

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[lmolkova]

@open-telemetry/semconv-mobile-approvers @MSNev @trask could you please take a look?

[MSNev]

I don't have any issue moving the event.name to the registry.