docs: fix return type's array shape for WP_Speculation_Rules::jsonSerialize()

[justlevine]

This PR fixes the return type on WP_Speculation_Rules::jsonSerialize() to show that it returns an array<string, mixed[]> instead of the internal array having a string key. This is due to the method's use of array_values() to intentionally strip any keys.

While this issue was surfaced via PHPStan in #7619 (trac: https://core.trac.wordpress.org/ticket/61175 ), it can be remediated independently of that ticket.

Trac ticket: https://core.trac.wordpress.org/ticket/63268

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[github-actions]

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[justlevine]

Not sure if this warrants a 6.8.x commit, but I definitely don't think it's worth bothering to create a Trac ticket just to make that happen.

[github-actions]

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[johnbillion]

How about array<string, array<int, mixed>>?

[justlevine]

Suggested change

	* @return array<string, mixed[]> Speculation rules data.
	* @return array<string, array<int ,mixed>> Speculation rules data.

[justlevine]

(GH autoresolved that on commit - repoened)