Upgrade Facebook Conversion API from v20 to v21

[Innovative-GauravKochar]

This Pull request is to upgrade API version from v20 to v21 in Facebook Conversion API.
Jira ticket :- https://segment.atlassian.net/browse/STRATCONN-5408
FlagOn Gate :- https://flagon.segment.com/families/centrifuge-destinations/gates/facebook-capi-actions-canary-version

Here is the Stage Testing Document.

Testing

Include any additional information about the testing you have completed to
ensure your changes behave as expected. For a speedy review, please check
any of the tasks you completed below during your testing.

• Added unit tests for new functionality
• Tested end-to-end using the local server
• [If destination is already live] Tested for backward compatibility of destination. Note: New required fields are a breaking change.
• [Segmenters] Tested in the staging environment
• [Segmenters] [If applicable for this change] Tested for regression with Hadron.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.50%. Comparing base (52537f3) to head (4c345c9).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2720      +/-   ##
==========================================
+ Coverage   78.46%   78.50%   +0.03%     
==========================================
  Files        1036     1036              
  Lines       18771    18772       +1     
  Branches     3561     3560       -1     
==========================================
+ Hits        14729    14737       +8     
+ Misses       2844     2836       -8     
- Partials     1198     1199       +1     

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