Bump onflow/go-ethereum dependency to v1.15.6

[m-Peter]

Work towards: #7152
Depends on: onflow/go-ethereum#20

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 59.70149% with 27 lines in your changes missing coverage. Please review.

Project coverage is 41.31%. Comparing base (6e6c6cd) to head (3da12b9).

Files with missing lines	Patch %	Lines
fvm/evm/emulator/state/stateDB.go	54.28%	16 Missing ⚠️
fvm/evm/types/call.go	0.00%	5 Missing ⚠️
fvm/evm/emulator/emulator.go	78.94%	4 Missing ⚠️
fvm/evm/emulator/state/delta.go	60.00%	2 Missing ⚠️

Additional details and impacted files

@@                    Coverage Diff                     @@
##           feature/pectra-upgrade    #7240      +/-   ##
==========================================================
- Coverage                   41.33%   41.31%   -0.02%     
==========================================================
  Files                        2164     2164              
  Lines                      189596   189633      +37     
==========================================================
- Hits                        78370    78355      -15     
- Misses                     104705   104748      +43     
- Partials                     6521     6530       +9     

Flag	Coverage Δ
unittests	41.31% <59.70%> (-0.02%)	⬇️

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[m-Peter]

Superseded by #7241