fix(dropdowns): forwards menu button ref correctly

[geotrev]

Description

This PR addresses a small bug with menu button ref handling, in addition to improving ref handling between buttonProps and button in a Menu.

Detail

The main fix is reversing Menu's ref (from forwardRef) from merging with triggerRef when composing button props. Instead, the ref from buttonProps is merged.

Now a consumer could achieve something like this:

<Menu buttonProps={{ ref: buttonRef }}>
  ...
</Menu>

... and the ref would be maintained. Compared to what a consumer would need to do to manage a ref today:

<Menu
  button={({ref, ...props}) => (
    <Button {...props} ref={mergeRefs([ref, buttonRef])}>Click me</Button>
  )}
>
  ...
</Menu>

I see the con here is worth considering, where a consumer must define a custom button any time they interact with the ref.

While a fix for handling modal focus with menus is under way, this change is meant to simplify the API a bit in general.

Checklist

• 👌 design updates will be Garden Designer approved (add the designer as a reviewer)
• 🌐 demo is up-to-date (npm start)
• ⬅️ renders as expected with reversed (RTL) direction
• ⚫ renders as expected in dark mode
• 🤘 renders as expected with Bedrock CSS (?bedrock)
• 💂‍♂️ includes new unit tests. Maintain existing coverage (always >= 96%)
• ♿ tested for WCAG 2.1 AA accessibility compliance
• 📝 tested in Chrome, Firefox, Safari, and Edge

[geotrev]

This ref is from Menu, which we don't want.

[ze-flo]

Not relying on consumers to use mergeRef is a win to me. 🙌