Remove the "51 Franklin Street, Fifth Floor, Boston" from the QEMU code base

The FSF recently moved away from "51 Franklin Street" (see e.g. https://www.fsf.org/about/contact/mailing for the current address). However, we still got that address listed in a bunch of GPL boiler plate text spread around the repository. This should get replaced, preferably with the URL to the license instead of using the new address, as it is recommended in the latest version of the GPLv2: https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt