First up, just s/buster/bullseye/g. See how much work we have to do to
get this building and working with Bullseye. Not anticipating much, but
you never know.

Second, update LEGAL file with SPDX data. Appears it's now required,
causing the build to fail. There isn't an exception that matches our
OpenSSL exception (but it close to openvpn-openssl-exception — that may
be where I got the language from). Update the copyright year, too.

Third, add perl-xs-dev. Appears to be needed to build now.