Fix gettransaction Bitcoin RPC endpoint

[brice-stacks]

The confirmations field should be a signed integer, since it goes negative when the transaction is conflicting.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.81%. Comparing base (fdf1041) to head (7c2cf19).
⚠️ Report is 4 commits behind head on develop.

❌ Your project check has failed because the head coverage (72.81%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6749      +/-   ##
===========================================
- Coverage    77.68%   72.81%   -4.88%     
===========================================
  Files          580      580              
  Lines       360194   360229      +35     
===========================================
- Hits        279832   262315   -17517     
- Misses       80362    97914   +17552     

Files with missing lines	Coverage Δ
...-node/src/burnchains/rpc/bitcoin_rpc_client/mod.rs	94.16% <ø> (-3.12%)	⬇️

... and 277 files with indirect coverage changes

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[simone-stacks]

LGTM!