fix: remove async/await usage to avoid babel polyfill

[gregjopa]

The bundle size did increase with in the latest releases due to using the async/await syntax and it getting polyfilled by babel.

https://bundlephobia.com/package/@paypal/react-paypal-js

This PR removes the async/await usage in favor of native Promises which reduces the overall bundle size (goes from 14 to 12 KB):

[codecov]

Codecov Report

Merging #150 (fbbe3d6) into main (a407cb8) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #150      +/-   ##
==========================================
+ Coverage   96.00%   96.05%   +0.05%     
==========================================
  Files          11       11              
  Lines         225      228       +3     
  Branches       68       68              
==========================================
+ Hits          216      219       +3     
  Misses          9        9              

Impacted Files	Coverage Δ
...rc/components/braintree/BraintreePayPalButtons.tsx	100.00% <100.00%> (ø)

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[borodovisin]

Sounds good I'll try to avoid the use of async/await in our code