feat: upgrade react to v18

[huniafatima-99]

Description

This PR upgrades react to v18 along with conversion of enzyme test suites to RTL.

Useful Links

Epic Link: edx/edx-arch-experiments#959
Issue Link: #446

[codecov]

Codecov Report

Attention: Patch coverage is 81.25000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 86.96%. Comparing base (729b659) to head (7dec9a9).

Files with missing lines	Patch %	Lines
src/index.jsx	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
+ Coverage   85.92%   86.96%   +1.04%     
==========================================
  Files         185      185              
  Lines        3872     3875       +3     
  Branches      948      958      +10     
==========================================
+ Hits         3327     3370      +43     
+ Misses        527      489      -38     
+ Partials       18       16       -2     

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