refactor: use maps.Copy to simplify the code

[oncecelll]

Description

There is a new function added in the go1.21 standard library, which can make the code more concise and easy to read.

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Author Checklist

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I have...

• tackled an existing issue or discussed with a team member
• left instructions on how to review the changes
• targeted the main branch

[aljo242]

@mattac21 to take a look at this. should be good - just want to make sure we have tests in place for this

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.60%. Comparing base (78502b5) to head (15c8e9b).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
mempool/txpool/txpool.go	33.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #925   +/-   ##
=======================================
  Coverage   65.59%   65.60%           
=======================================
  Files         318      318           
  Lines       22256    22253    -3     
=======================================
- Hits        14599    14598    -1     
+ Misses       6486     6484    -2     
  Partials     1171     1171           

Files with missing lines	Coverage Δ
mempool/txpool/txpool.go	47.23% <33.33%> (+0.20%)	⬆️

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[mattac21]

do you mind adding tests for these? should be pretty straightforward, looks like we already have a mocked subpool you can use here. thanks!