Productivity Analytics: Add getter for scatterplotStartDate

What does this MR do?

With the changes for the Productivity Analytics endpoint in !21031 (merged) we will add param validation.

This MR ensures that when FE requests data for the scatterplot, the merged_at_after date is not before a the minDate (provided it exists), otherwise the API responds with 422.

Implementation

• The minDate is being set on the state.
• A scatterplotStartDate getter is added which computes the date based on the selected startDate and the given minDate
• The scatterplot endpoint gets called with the computed scatterplotStartDate

Screenshots

N/A

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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