Deprecate the `addslashes_gpc()` function

[johnbillion]

The addslashes_gpc() function is just a wrapper for wp_slash(). It should be moved to deprecated.php and deprecated to help reduce confusion about magic quoting, which is where this function originated.

There are some uses of addslashes_gpc() that need to be switched to wp_slash() too.

In this pull request, we are deprecating the addslashes_gpc function as it was just a wrapper function to wp_slash
Additionally, I have added PHPUnit tests for the same as well, let me know if they are not required and I can remove it.

Trac ticket: https://core.trac.wordpress.org/ticket/64539

@audrasjb @westonruter @mindctrl Thanks for the reviews.
Who would be the best person to merge this to the SVN?

I'm happy to do so.

[westonruter]

In 61590:

Formatting: Deprecate the addslashes_gpc() function.

This deprecates addslashes_gpc() in favor of wp_slash(), as the former is just a wrapper for the latter. The three remaining uses of addslashes_gpc() (in WP_Query) have been replaced with wp_slash(). Unit tests are added to verify that they have the same behavior.

Developed in ​https://github.com/WordPress/wordpress-develop/pull/10771

Follow-up to [23591], [23555].

Props rutviksavsani, audrasjb, westonruter, mindctrl, johnbillion.
See #21767.
Fixes #64539.

[audrasjb]

This is worth mentioning in the Field Guide.
Also adding needs-dev-note just in case we can mention it in a more general note.

[westonruter]

I drafted a dev note section for the field guide: ​https://docs.google.com/document/d/1PpVQE9kuNE0KWfQxUQKfrbzfuRBkuSg6Sz7-NUWCsKI/edit?tab=t.0

I wanted to take the opportunity to provide context for why this function was needed and why wp_slash() is still needed, since authors can still liable to pass unslashed data to functions that require it.

Related: ​https://github.com/WordPress/WordPress-Coding-Standards/issues/172