Remove godoc.org references

[oliverpool]

I went over the README to check all godoc.org references.

I removed the libraries which apparently do not meet the quality standard:

• missing tagged version:
  • https://godoc.org/github.com/tvastar/test/cmd/testmd
  • https://godoc.org/github.com/shurcooL/github_flavored_markdown
• outdated:
  • https://github.com/posener/tarfs (no support for io.FS, implements the github.com/kr/fs#FileSystem (and not the stdlib io.FS interface), package which has been replaced by kr.dev/walk)
  • gowalker.org does not serve Go content

For the others, I replaced the godoc.org link with a pkg.go.dev link.

[github-actions[bot]]

Thank you for contributing to awesome-go. We will review your contribution as soon as possible.

Make sure you add the links in the body of the pull request that are requested in the contribution guide:

• repo link
• pkg.go.dev
• goreportcard.com
• coverage

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