Refactor: Reuse v6 BlockTransactionCount handler for v7 and v8

[hudem1]

rpc pkg cleanup according to #2437

Reuse v6 BlockTransactionCount handler both for v7 and v8 starknet_getBlockTransactionCount endpoint

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.50%. Comparing base (bd16b82) to head (1b95b86).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2488      +/-   ##
==========================================
- Coverage   73.53%   73.50%   -0.04%     
==========================================
  Files         140      140              
  Lines       17147    17130      -17     
==========================================
- Hits        12609    12591      -18     
  Misses       3654     3654              
- Partials      884      885       +1     

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