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().

Trac ticket: https://core.trac.wordpress.org/ticket/36399

[flixos90]

I've opened a ​pull-request with an updated version of this change. It also includes:

• tests to cover both new and legacy usage of wp_count_terms()
• all references in core code have been updated to use new signature

As originally planned, because this function is widely used and there is no actual drawback to use the old signature, legacy usage shouldn't trigger a deprecated notice (similar like with get_terms()). Yet, proceeding with this change makes sense for consistency and to make it obvious that wp_count_terms() can also count all terms, not only terms of certain taxonomies.

[flixos90]

In 48840:

Taxonomy: Allow for wp_count_terms( $args ) signature, making passing a taxonomy optional.

This brings wp_count_terms() in line with other taxonomy functions such as get_terms() which technically no longer require a taxonomy. Similar to the previously modified functions, no deprecation warning is triggered when using the legacy signature.

Fixes #36399.

[SergeyBiryukov]

In 48843:

Docs: Update the description for the $legacy parameter of wp_count_terms() for consistency with get_terms().

Follow-up to [48840].

See #36399.