Productivity Analytics: Align custom tooltip styling for scatterplot

What does this MR do?

In gitlab-ui!867 (merged) we've standardized our default tooltips for gitlab-ui charts. This MR adjusts the custom tooltip for the scatterplot to be aligned with our default tooltip format.

Note: The reason for us using a custom tooltip in this case is because we want to display the date plus time portion in the tooltip (as opposed to just showing the date).

This is behind the productivity_analytics feature flag.

Screenshots

Before	After

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