[BUG] Update reporting menu item UI to support variable header heights

[joshuarrrr]

What is the bug?
In the 2.1.0 release of Opensearch Dashboards, we're planning to add a configurable option for either a condensed (single-line) or expanded (2-line) global header. However, this plugin currently hard-codes the header height into the popover positioning, which means the popover will no longer align with the menu link in the default mode:

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Pull the feature branch of opensearch dashboards, or simply change the multiplier here to 1.
2. Load opensearch dashboards with the dashboard-reports plugin and click on the Reporting menu link.

What is the expected behavior?
Because the plugin is responsible for rendering both the menu link and popover, it should use the correct EUI components, which will take care of positioning the popover relative to the menu link. Instead of building up large HTML strings with template literals, using EUI React components will make the UI controls much more maintainable and standardized.