Change function signature of `wp_count_terms()` to be compliant with recent `get_terms()` changes

[flixos90]

In #35495 the $taxonomy parameter of get_terms() was made optional, thus changing the function signature to the first parameter being the $args array where the taxonomy (if needed) could then be added to.

This ticket is about changing the signature of wp_count_terms() in a similar manner. This will bring more consistency and it will also allow to count all terms entirely (if no taxonomy is provided).

[flixos90]

36399.diff​ is a patch for this. Specifically we need to determine if the check for $do_legacy_args is safe enough here. As an alternative, we could create a new internal function like _get_terms_defaults() which would allow us to do the same kind of verification as in get_terms().