* rewrite `just update` command to provide a one-line command to update everything

* update prefix

* uncomment update-self

* Revert requirements.yml updates not belonging to this PR

* Justfile and documentation updates to make things clearer

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Co-authored-by: Slavi Pantaleev <slavi@devture.com>

We keep the Makefile for now, but don't mention it to new
users anymore.