Update XMLHttpRequest.getAllResponseHeaders() implementation (#32353) (#32363)

Summary:
As per the XMLHttpRequest specification [1], getAllResponseHeaders() should return a string of headers with lowercased names and sorted by their uppercase representation, with each header ending with '\r\n'.

[1] https://xhr.spec.whatwg.org/#the-getallresponseheaders()-method

## Changelog

[General] [Changed] XMLHttpRequest.getAllResponseHeaders() now returns headers with names lowercased and sorted in ascending order, as per specification

Pull Request resolved: https://github.com/facebook/react-native/pull/32363

Test Plan:
Test derived from Web Platform Test repository:
https://github.com/web-platform-tests/wpt/tree/master/xhr

Reviewed By: yungsters

Differential Revision: D31626217

Pulled By: sota000

fbshipit-source-id: 299d005facbe1c15b8cda5eed6750db75addca80