feat: remove UpgradeButton

[MaxFrank13]

This removes the UpgradeButton component (#438):

• UpgradeButton is no longer the default content in the CourseCardActionSlot,
• Removes defintion and test files (ie. UpgradeButton.(test.)jsx)
• Removes tracking functions and any other logging utilities specific to the UpgradeButton, including event names and onClick callbacks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.73%. Comparing base (84b281a) to head (9529da8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #536      +/-   ##
==========================================
- Coverage   97.40%   96.73%   -0.68%     
==========================================
  Files         157      157              
  Lines        1389     1377      -12     
  Branches      231      229       -2     
==========================================
- Hits         1353     1332      -21     
- Misses         34       43       +9     
  Partials        2        2              

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