fix: sankey types - add expliciy return type for renderTooltip

[ckifer]

Description

#3580

Broken types after refactoring Tooltip to a function component because renderTooltip of Sankey didn't explicitly set a return type. This confirms the flakiness of recharts types and the use of React.cloneElement.

Before any more refactoring changes try to build into a project and compile it with tsc

Related Issue

#3580

Motivation and Context

How Has This Been Tested?

• reproduce error with test repo
• look at Sankey.d.ts in recharts build after tsc -> see error in output
• look at implied type of renderTooltip
• look at other renderTooltip functions (they all use React.ReactElement)
  • see search https://github.com/search?q=repo%3Arecharts%2Frecharts%20renderTooltip&type=code
• explicitly set type returned to React.ReactElement

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.