The PHP native JSON extension has been bundled and compiled with PHP by default since version 5.2.0. Because the minimum version of PHP required by WordPress is now 5.6.20 (see #46594 and [45058]), JSON extension related polyfills and backwards compatibility code can now be removed.
This change removes code that supported JSON related functionality on older versions of PHP. This includes (but is not limited to) checks that `json_last_error()` exists, checking and setting the `JSON_UNESCAPED_SLASHES` and `JSON_PRETTY_PRINT` constants if not previously defined, and deprecating the `_wp_json_prepare_data()` function (which was 100% workaround code).
Follow up of [46205].
See #47699.
Props jrf, Clorith, pento.
git-svn-id: https://develop.svn.wordpress.org/trunk@46206 602fd350-edb4-49c9-b593-d223f7449a82
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