fix(Dropdown): update typings for defaultValue & value

[zhongeric]

Fixed typings issue for defaultValue and value in Dropdown.d.ts

Fixes #4017

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💖 Thanks for opening this pull request! 💖

Here is a list of things that will help get it across the finish line:

• Run yarn lint locally to catch formatting errors. This will fix some errors automatically, commit and push any changes.
• Run yarn test locally to catch errors. This ensures all components still behave as they should.
• Run yarn start to run the doc site locally and try a few pages, ensuring everything is in good working order.
• Include tests when adding/changing behavior.

We get a lot of pull requests on this repo, so please be patient and we will get back to you as soon as we can.

[codecov-commenter]

Codecov Report

Merging #4018 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4018   +/-   ##
=======================================
  Coverage   99.84%   99.84%           
=======================================
  Files         185      185           
  Lines        3251     3251           
=======================================
  Hits         3246     3246           
  Misses          5        5           

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[layershifter]

❤️

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉