Clarify the license of erb

[k0kubun]

ref: ruby/resolv#45

[k0kubun]

Patch authors since 86e24c8:

$ git shortlog -sn 86e24c83f5293e6c1f8bdb210b43e831b2c87bff..
76 Takashi Kokubun
15 Hiroshi SHIBATA
2 dependabot[bot]
1 Jean Boussier
1 Josh Nichols
1 Kenichi Kamiya
1 Lars Kanis
1 Nobuyoshi Nakada
1 b-sakata
1 ooooooo_q

@hsbt@byroot@technicalpickles@kachick@larskanis@nobu@b-sakata@ooooooo-q If you object to including 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.

[k0kubun]

Given that @nobu and @b-sakata didn't reply in time, I assume they have no objection to the change. Everybody else has made an explicit 👍 reaction (image attached for the record).