Use request_parse_body() in PHP 8.4 and remove special handling of input stream file uploads

Problem/Motivation

PHP 8.4 introduces a request_parse_body() function that reads the php://input stream and populates the $_POST and $_FILES superglobals, such as when you upload files via a form with "multipart/form-data" enctype.

We should see if we can use this and avoid special handling for REST and JSON-API file uploads.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet