Clarify the license of net-smtp

[tmtm]

ruby/resolv#45 realized me that LICENSE.txt contains only BSD-2-Clause license, while gemspec specifies both Ruby's and BSD-2-Clause license. So I'd like to clarify the license.

[tmtm]

Patch authors since https://github.com/ruby/net-smtp/tree/v0.1.0 (df67714)

% git shortlog -sn v0.1.0..
    82	TOMITA Masahiro
    31	Hiroshi SHIBATA
     5	Nobuyoshi Nakada
     5	nick evans
     3	Arnt Gulbrandsen
     3	Kazuhiro NISHIYAMA
     3	Olle Jonsson
     2	Akinori MUSHA
     2	Jeremy Evans
     2	Kazuki Yamaguchi
     2	Sorah Fukumori
     2	Takashi Kokubun
     2	Tom Freudenberg
     2	Yusuke Endoh
     2	Yuta Saito
     2	dependabot[bot]
     1	Akira Matsuda
     1	Benoit Daloze
     1	Jean Boussier
     1	MSP-Greg
     1	Mantas
     1	Peter Goldstein
     1	SHIBATA Hiroshi
     1	krororo
     1	m-nakamura145
     1	mohamed
     1	nicholas a. evans
     1	ojab

@hsbt @nobu @nevans @arnt @znz @olleolleolle @knu @jeremyevans @rhenium @sorah @k0kubun @TomFreudenberg @mame @kateinoigakukun @amatsuda @eregon @byroot @MSP-Greg @mantas @petergoldstein @krororo @m-nakamura145 @mohamedhafez @nevans @ojab
If you object to including the Ruby's license, could you mention it here by 2024-06-01 00:00:00 UTC?
Just add a 👍 reaction, if you have no objection.