Update GPLv2 text for remote-only FSF

[musicinmybrain]

The Free Software Foundation is now remote-only, and no longer has a physical mailing address. This PR updates COPYING with the latest text from https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt, while taking care to preserve the added notice at the end:

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Note:
This is a GNU GPL "v2 only" license. 
This is not a GNU GPL "v2 or any later version" license.
                                                             ---Martin Blais