tools: fix license-builder.sh again for ICU

* Modify tools/license-builder.sh to support ICU 57.1's plain text
license. (Separate issue to add ICU 57.1 in #6058)
* Update/regenerate LICENSE to include ICU 57.1's license
* Note that because the tool was rerun, the change in #6065 is already
included here.

PR-URL: #6068
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: James M Snell <jasnell@gmail.com>