Performance improvements for productivity analytics scatterplot

What does this MR do?

Improves performance when computing the median line data for the scatterplot on the Productivity Analytics page by leveraging the utility methods getScatterPlotDataNew and getMedianLineDataNew introduced in !18314 (merged)

With this improvement we will group MRs by date on the scatterplot, i.e., MRs which got merged on the same date will be stacked on the y axis since we now use date instead of dateTime on the x axis. In addition, we now compute the median line only per date and don't need to loop over all dateTimes.

This is behind the productivity_analytics_scatterplot feature flag.

Closes #33346 (closed)

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team