External Libraries: Update the SimplePie library to the latest version (1.5.5).

This brings SimplePie in sync with the most up to date version, 1.5.5.

This update brings many bug fixes, small enhancements, and PHP compatibility fixes for newer versions of PHP.

For a full list of changes, see https://github.com/simplepie/simplepie/blob/master/CHANGELOG.md#155-may-1-2020.

Props dshanske, slushman, etruel, wpshades, dmenard, desrosj, hareesh-pillai, stevenkword, jrf, Ipstenu, johnbillion.
Fixes #36669.
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git-svn-id: https://core.svn.wordpress.org/trunk@47509 1a063a9b-81f0-0310-95a4-ce76da25c4cd