docs: Revise update-and-cleanup.md

[jpduyx]

spotify dockergc is UNMAINTAINED, they advice to consider using docker system prune command instead.

"This repository has been archived by the owner on Feb 2, 2021. It is now read-only."

https://github.com/spotify/docker-gc

Description

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change that does improve existing functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[polarathene]

LGTM 👍

Thanks for spotting this and contributing an update. I'll apply these review suggestions myself :)

[github-actions]

Documentation preview for this PR is ready! 🎉

Built with commit: b3fbe8f

[polarathene]

Before (v12.1)

After (v13)

[jpduyx]

thank you :-)