refactor: use slices.Contains to simplify code

[clonemycode]

Summary

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

Test Plan

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 50.46%. Comparing base (15a7913) to head (3d1ddd7).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/util/cmd.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6357      +/-   ##
==========================================
- Coverage   50.63%   50.46%   -0.17%     
==========================================
  Files         653      653              
  Lines      110343   110342       -1     
==========================================
- Hits        55872    55685     -187     
- Misses      51601    51796     +195     
+ Partials     2870     2861       -9     

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[Copilot]

Pull Request Overview

This PR refactors membership checks to use the new slices.Contains function from Go 1.21, making the code more concise and readable.

• Replace manual for loops in phonebook_test.go with slices.Contains
• Simplify the Set method in cmd/util/cmd.go by using slices.Contains
• Add slices to the import blocks where needed

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
network/phonebook/phonebook_test.go	Replaced nested loops with slices.Contains in tests
cmd/util/cmd.go	Simplified allowed-value check in CobraStringValue.Set()

Comments suppressed due to low confidence (1)

go.mod:1

• Since the code now imports the standard-library slices package (added in Go 1.21), please update the go directive in go.mod to 1.21 to ensure compatibility.

module github.com/algorand/go-algorand

[clonemycode]

Thanks for the approved.