docs: show NEXT in version selectors

[mdjermanovic]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[x] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

Enables NEXT in version selectors.

What changes did you make? (Give an overview)

• Set config.showNextVersion to true to enable NEXT in version selectors (see 6669413).
• Updated eslintVersion to read from config.version, because pkg.version will soon become 9.0.0-alpha.0.

Is there anything you'd like reviewers to focus on?

• If we merge this now, NEXT will appear in the lists on https://eslint.org/docs/head before the release. Is that okay?
• When this is merged, I'll sync the next branch.
• I'll send a separate PR targeting the latest branch to set "showNextVersion": true there.
• eslintVersion also reads from ESLINT_VERSION environment variable, so we could just set this variable on Netlify instead of hardcoding the latest version in the config file. However, this is a temporary solution anyway, and this will make it more clear where the 8.56.0 value is coming from when we start working on a long-term solution.

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[nzakas]

LGTM. I'd say let's hold off on merging this until we do the release. That should (I think?) update the next branch.

[mdjermanovic]

Yes, we could merge this right before the release, and the next branch will include this commit when the branch gets updated during the release. I'm switching to draft so that we don't accidentally merge this before the time comes.