docs: Add RuleTester changes to migrate to v9 guide

[nzakas]

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:

What changes did you make? (Give an overview)

Updated the "Migrate to v9" guide to include RuleTester changes.

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

[eslint-github-bot]

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