Set Default version to v21 in Facebook Conversion API

[Innovative-GauravKochar]

Clean up and Rollout out of v20.0 APIs:#2720
This PR is to set Default Version to V21 .
Jira ticket :- https://segment.atlassian.net/browse/STRATCONN-5408

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 ✅

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2740   +/-   ##
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  Coverage        ?   78.49%           
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  Files           ?     1036           
  Lines           ?    18798           
  Branches        ?     3571           
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  Hits            ?    14756           
  Misses          ?     2837           
  Partials        ?     1205           

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