crypto/secp256k1: unify the package license to 3-Clause BSD (#17225)

Our original wrapper code had two parts. One taken from a third
party repository (who took it from upstream Go) licensed under
BSD-3. The second written by Jeff, Felix and Gustav, licensed
under LGPL. This made this package problematic to use from the
outside.

With the agreement of the original copyright holders, this commit
changes the license of the LGPL portions of the code to BSD-3:

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I agree changing from LGPL to a BSD style license.

Jeff
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Hey guys,

My preference would be to relicense to GNUBL, but I'm also OK with BSD.

Cheers,
Gustav
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Felix Lange (fjl):
I would approve anything that makes our licensing less complicated
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