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
• [x] 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[bot]]

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 :warning:
pkg/components/page/page.go	0.00%	3 Missing :warning:
internal/providers/gofpdf/text.go	0.00%	1 Missing :warning:
pkg/merge/merge.go	66.67%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/johnfercher/maroto/pull/384/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+Fercher) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/johnfercher/maroto/pull/384/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+Fercher) | `79.84% <30.77%> (-0.04%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+Fercher#carryforward-flags-in-the-pull-request-comment) to find out more.

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[houseme]

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

👌