Chore: Update product_group knowledge_graph to context_systems

What does this MR do and why?

Update product group knowledge_graph to context_systems

Updates product group of events and metrics from knowledge_graph (no longer valid) to context_systems

This will result in less invalid events, as per our alert.

References

Alert: https://gitlab.enterprise.slack.com/archives/C072281EM5E/p1784534630826409

Current product groups: https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/data/stages.yml

MR acceptance checklist

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