COPYING: replace FSF postal address with licenses URL

COPYING: replace FSF postal address with licenses URL

The license text in COPYING (GPLv2), COPYING.LIB (LGPLv2.1),
and the linux-headers/LICENSES/preferred/GPL-2.0 file are
referenced to the obsolete FSF postal address.

Replace it with the canonical GNU licenses URL recommended by the FSF:
https://www.gnu.org/licenses/

Signed-off-by: Sean Wei <me@sean.taipei>
Message-ID: <20250613.qemu.patch.01@sean.taipei>
[thuth: dropped the changes to the linux-headers folder]
Signed-off-by: Thomas Huth <thuth@redhat.com>