COPYING.LESSER: Replace LGPLv3 with LGPLv2.1.
COPYING.LESSER: Replace LGPLv3 with LGPLv2.1.

The file incorrectly contained a copy of the LGPL, version 3. It should
have contained version 2.1 though, since that's the license of the
fx2lib files it applies to.

uffejakobsen/sigrok-firmware-fx2lafw,swegener/sigrok-firmware-fx2lafw,swegener/sigrok-firmware-fx2lafw,uffejakobsen/sigrok-firmware-fx2lafw,swegener/sigrok-firmware-fx2lafw,swegener/sigrok-firmware-fx2lafw,uffejakobsen/sigrok-firmware-fx2lafw,uffejakobsen/sigrok-firmware-fx2lafw