refactor(storagenode): remove synchronous Append

[ijsong]

What this PR does

This PR removes the synchronous Append method from Executor, which is unused
code. It also updates tests to use the asynchronous AppendAsync method and
ensures proper handling of AppendTask in tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.02%. Comparing base (b74daea) to head (4f23b35).

Additional details and impacted files

@@                                         Coverage Diff                                          @@
##           refactor_sn_committer_to_remove_requirecommitwaittasks_parameter     #970      +/-   ##
====================================================================================================
- Coverage                                                             80.19%   80.02%   -0.17%     
====================================================================================================
  Files                                                                   178      178              
  Lines                                                                 21500    21432      -68     
====================================================================================================
- Hits                                                                  17241    17151      -90     
- Misses                                                                 3477     3495      +18     
- Partials                                                                782      786       +4     

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