feat: improve code and comments, uniformly upgrade the versions of setup-go and checkout in the action operation.

[houseme]

Description

1.Improve code and comments, uniformly upgrade the versions of setup-go and checkout in the action operation.
2.Upgrade pdfcpu version to v0.6.0
3.Execute go mod tidy to remove irrelevant indirect dependencies

Related Issue

#386

Checklist

check with "x", if applied to your change

• All methods associated with structs has func (<first letter of struct> *struct) method() {} name style.
• Wrote unit tests for new/changed features.
• Followed the unit test when,should naming pattern.
• Updated docs/doc.go and docs/*
• Updated pkg/pdf/example_test.go
• Updated README.md
• Executed make examples to update all examples inside docs/examples.
• New public methods/structs/interfaces has comments upside them explaining they responsibilities
• Executed make dod with none issues pointed out by golangci-lint

[johnfercher]

Increadible your PR, thank you for the contribution. I will review it better soon.

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (2dd0925) 79.89% compared to head (7bf9c98) 79.84%.

Files	Patch %	Lines
pkg/metrics/metrics.go	0.00%	4 Missing ⚠️
pkg/components/page/page.go	0.00%	3 Missing ⚠️
internal/providers/gofpdf/text.go	0.00%	1 Missing ⚠️
pkg/merge/merge.go	66.67%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #384      +/-   ##
==========================================
- Coverage   79.89%   79.84%   -0.04%     
==========================================
  Files          61       61              
  Lines        2018     2018              
==========================================
- Hits         1612     1611       -1     
- Misses        378      379       +1     
  Partials       28       28              

Flag	Coverage Δ
unittests	79.84% <30.77%> (-0.04%)	⬇️

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[johnfercher]

Could you fix these comments? They are mostly copy & paste.

[houseme]

Could you fix these comments? They are mostly copy & paste.

👌