Migrate to urfave-cli v3

[majst01]

Description

This must be done, but does not work somehow, envvars are not evaluated ?

Bug in urfave/cli/v3: urfave/cli#2041 seems fixed now

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 79 lines in your changes missing coverage. Please review.
✅ Project coverage is 34.12%. Comparing base (6130564) to head (832b142).

Files with missing lines	Patch %	Lines
cmd/server/main.go	0.00%	46 Missing ⚠️
cmd/client/client.go	0.00%	33 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
- Coverage   34.15%   34.12%   -0.04%     
==========================================
  Files          17       17              
  Lines        3124     3127       +3     
==========================================
  Hits         1067     1067              
- Misses       1835     1838       +3     
  Partials      222      222              

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