fix: incorrect array access to template_name property in get_taxonomy_labels()

[justlevine]

Trac ticket:
https://core.trac.wordpress.org/ticket/64238
https://core.trac.wordpress.org/ticket/63268

This PR fixes an issue in get_taxonomy_labels(), where the template_name was accessed as an object property on $tax->labels instead of as an array property.

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.

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[github-actions]

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[justlevine]

Not sure why this is breaking things. $tax->labels is coerced into an array at the top of the function 🤔

[github-actions]

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[justlevine]

On further investigation, this is invalid, and labels is indeed reconverted back to an object.