MPE: Replace Preference -> Planning

[taneliang]

Context

#3035. Forgot to fix these names after RO clarified that it's a planning exercise, not a preference exercise.

[vercel]

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[codecov]

Codecov Report

Merging #3253 (16e9b63) into master (f23300a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3253   +/-   ##
=======================================
  Coverage   52.77%   52.77%           
=======================================
  Files         270      270           
  Lines        5720     5720           
  Branches     1319     1319           
=======================================
  Hits         3019     3019           
  Misses       2701     2701           

Impacted Files	Coverage Δ
website/src/types/modules.ts	100.00% <ø> (ø)

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[nusmods-deploy-bot]

Deployment preview for 16e9b639 ready at http://603c2f9c060121180f536a95--nusmods-deploy-preview.netlify.app