Upgrade to eslint `v9`

[felipeqq2]

ESLint released v9, with many breaking changes. I can't make it work with eslint-plugin-ava.

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There may be other issues, but the ones I found were:

• The eslintrc configuration format was deprecated in favor of "flat config files";
• ava/prefer-t-regex is using context.getScope, now removed (feat!: Remove deprecated context methods eslint/eslint#17698).