feat: update frontend-lib-learning assistant version 2.20.0 2.21.1

[mraman-2U]

This pull request includes updates to dependencies and asset references in the codebase. The most important changes involve upgrading the @edx/frontend-lib-learning-assistant dependency and replacing references to certificate assets with updated versions.

Dependency Updates:

• package.json: Upgraded @edx/frontend-lib-learning-assistant from version ^2.20.0 to ^2.21.1.

Asset Reference Updates:

• src/courseware/course/course-exit/CourseCelebration.jsx: Updated certificate asset references from openedx_certificate.png and openedx_locked_certificate.png to edX_certificate.png and edX_locked_certificate.png.
• src/courseware/course/course-exit/ProgramCompletion.jsx: Updated certImage reference from openedx_certificate.png to edX_certificate.png.
• src/courseware/course/sequence/lock-paywall/LockPaywall.jsx: Updated certificateLocked reference from openedx_locked_certificate.png to edX_locked_certificate.png.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.46%. Comparing base (7eddc91) to head (510df1c).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1760      +/-   ##
==========================================
+ Coverage   90.45%   90.46%   +0.01%     
==========================================
  Files         344      344              
  Lines        5804     5812       +8     
  Branches     1357     1360       +3     
==========================================
+ Hits         5250     5258       +8     
  Misses        537      537              
  Partials       17       17              

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[ilee2u]

LGTM 🚀