Code Modernisation: Introduce the spread operator in `Walker::walk()` and `::paged_walk()`.

Rather than relying on `func_get_args()` to retrieve arbitrary function arguments, we can now use the spread operator to assign them directly to a variable.

This re-applies [45624] and reverts [45640], with a dev note upcoming for plugin authors to maintain backward compatibility with old versions of WordPress.

Props jrf.
See #47678.
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