fix: update CONTRIBUTING.md documentation for mac setup, bump node version, fix rubocop error

[jamesoneill997]

Wanted to get this setup locally on mac and ran into a couple of non-intuitive issues.

Tested and working on MacOS Sequoia Version 15.6.1.

[jamesoneill997]

Rubocop 1.2 is a pretty old base to have. It looks like all the other PRs are failing CI because of the name change introduced in v1.76.0
https://github.com/rubocop/rubocop/releases/tag/v1.76.0

idk if you will also want to bump the minimum rubocop version to 1.76 also?

Actions CI is running rubocop v1.80.2 (at the time of writing, the latest version), that would probably benefit from being anchored to the same version as whatever the minimum is in the Gemfile?

Node needs updating too for the rest of the CI.
The engine "node" is incompatible with this module. Expected version "20 || >=22". Got "18.20.8"

Might be better for me to open a separate PR, I just wanted to update the docs 🙃

Though I might just handle it in this PR, as it's in a bit of a state.

[jamesoneill997]

I'd imagine that remaining failure is to do with importmap stuff.

     Failure/Error: imports['@popperjs/core'].gsub!('lib/index.js', 'dist/esm/popper.js')

     NoMethodError:
       undefined method `gsub!' for nil:NilClass

[jamesoneill997]

Same issue as #3694 it seems?

[mshibuya]

Merged in. Thank you!