[DOP-23979] Change default FileDFWriter.Options(if_exists) to APPEND

[dolfinus]

Change Summary

Make default value of FileDFWriter.Options(if_exists=...) consistent with DB connections, which use APPEND.

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.97%. Comparing base (60aea9e) to head (f71a402).
Report is 67 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #343      +/-   ##
===========================================
- Coverage    92.02%   91.97%   -0.06%     
===========================================
  Files          228      228              
  Lines         9867     9867              
  Branches      1013     1013              
===========================================
- Hits          9080     9075       -5     
- Misses         599      602       +3     
- Partials       188      190       +2     

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