activerecord: Add type for AR::QueryMethods::WhereChain#associated

[tk0miya]

ActiveRecord::QueryMethods::WhereChain#associated has been added since v7.0.

refs: rails/rails#40696

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activerecord

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[ksss]

APPROVE

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Thanks for your review, @ksss!

@tk0miya, @ksss This PR is ready to be merged.
Just comment /merge to merge this PR.

[tk0miya]

Thanks!

[tk0miya]

/merge