refactor: use the built-in max/min to simplify the code [skip changelog]

[overallteach]

In Go 1.21, the standard library includes built-in max/min function, which can greatly simplify the code.

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[overallteach]

Not LLMs. This change can make the code more clean.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 61.59%. Comparing base (6d9782e) to head (887767f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
gateway/backend_blocks.go	0.00%	1 Missing ⚠️

@@            Coverage Diff             @@
##             main     #941      +/-   ##
==========================================
- Coverage   61.59%   61.59%   -0.01%     
==========================================
  Files         254      254              
  Lines       31400    31396       -4     
==========================================
- Hits        19341    19337       -4     
+ Misses      10487    10486       -1     
- Partials     1572     1573       +1     

Files with missing lines	Coverage Δ
ipld/merkledag/coding.go	77.86% <100.00%> (-0.17%)	⬇️
gateway/backend_blocks.go	46.40% <0.00%> (+0.23%)	⬆️

... and 9 files with indirect coverage changes

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