Update Corepack installation instructions

[fulldecent]

Clarified the opt-in status of Corepack and noted its removal from future Node.js releases.

What's the problem this PR addresses?

Corrects lanuage about the future of Corepack.

How did you fix it?

Cited authoritative sources.

Checklist

• [x] I have read the Contributing Guide.

• [x] I have set the packages that need to be released for my changes to be effective.

• [x] I will check that all automated PR checks pass before the PR gets reviewed.