Removing PHP4 backward compatibility in do_action()

[psaikali]

https://core.trac.wordpress.org/ticket/55133

Removing the PHP4 backward-compatibility in the do_action() function to avoid unexpected behaviour where the first argument is an array of a single object, as it's currently flattened and become a variable containing the single object.

Trac ticket: [](https://core.trac.wordpress.org/ticket/55133)

[github-actions]

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