Get rid of bogus use of unicode package, go back to only require go 1.20

[ldemailly]

I thought I did this in #7 but didn't / got side tracked so here it is

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fb8b190) to head (3d170ac).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #8   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          189       195    +6     
=========================================
+ Hits           189       195    +6     

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[Copilot]

Pull Request Overview

This PR removes the dependency on the unicode package and reverts the Go version requirement from 1.21 back to 1.20. The changes replace unicode package functions with custom helper functions and substitute the built-in min function (introduced in Go 1.21) with explicit comparison logic.

• Replaces unicode.IsLetter() with custom character checking logic
• Substitutes min() function with manual comparison
• Adds helper functions for character classification

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
go.mod	Downgrades Go version requirement from 1.21 to 1.20
duration.go	Removes unicode import, adds helper functions, and replaces min() with manual comparison

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