Rename gas_fee to max_gas_fee

[gurukamath]

What was wrong?

Post London, the gas_fee calculated in fork.py in in fact the max gas that can be paid in the transaction. The variable is renamed accordingly.

Related to Issue #877

How was it fixed?

Renamed the relevant variable.

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[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (9ca885c) 74.12% compared to head (30695b3) 74.12%.

Files	Patch %	Lines
src/ethereum/arrow_glacier/fork.py	0.00%	3 Missing ⚠️
src/ethereum/gray_glacier/fork.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #886   +/-   ##
=======================================
  Coverage   74.12%   74.12%           
=======================================
  Files         572      572           
  Lines       32419    32419           
=======================================
  Hits        24030    24030           
  Misses       8389     8389           

Flag	Coverage Δ
unittests	74.12% <60.00%> (ø)

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