license: Move to BSD-2-Clause and drop -FreeBSD

[imp]

Summary

BSD-2-Clause-FreeBSD has been deprecated as a separate license. Move to
the supported BSD-2-Clause since that more closely matches our
license. The -FreeBSD version SPDX landed on also doesn't match our
preferred license as well.

Sponsored by: Netflix

[andrew]

https://tools.spdx.org/app/check_license/ also thinks the license text is BSD-2-Clause

[pauamma_gundo.com]

As long as the changed license ID matches the text (and per comment above, SDPX thinks it does), it's good enough for not-a-lawyer me.