Change analytics dashboards data sources back to functions

What does this MR do and why?

Revert analytics dashboards data sources refactor

• revert back from class based data sources to floating fetch fn
• initial reason to move to classes is not longer required, so simpler function based implementation is desireable

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

No visual differences.

How to set up and validate locally

1. Follow these instructions to setup Product Analytics in GDK.
2. Onboard Product Analytics by setting up the feature at Project > Analytics > Analytics dashboards.
3. Seed value_stream data if necessary docs
4. Verify that analytics dashboards work as expected:

• cube based product analytics
• dora_metrics_over_time (validation instructions)
• usage_overview
• value_stream
• check project-level + group-level VSD dashboards

Relates to #464123 (closed)