opt: bump up minimum required Go version to 1.20

[panjf2000]

Employ the standard approach of string<->bytes conversion

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It's rational to bump the minimum required Go version from 1.17 to 1.20 at this stage since Go 1.21 has been EOL. Check out https://endoflife.date/go

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.67%. Comparing base (18f54e3) to head (20b5a50).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #638      +/-   ##
==========================================
+ Coverage   80.34%   80.67%   +0.33%     
==========================================
  Files          21       21              
  Lines        2096     2096              
==========================================
+ Hits         1684     1691       +7     
+ Misses        285      282       -3     
+ Partials      127      123       -4     

Flag	Coverage Δ
unittests	80.67% <ø> (+0.33%)	⬆️

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