Migrate to SPDX license
    
    CC0-1.0 was discussed specifically for libb2 on fedora-legal mailing list in August. Note that is allowed by exception:

      Existing uses of CC0-1.0 on code files in Fedora packages prior to

      2022-08-01, and subsequent upstream versions of those files in those

      packages, continue to be allowed. We encourage Fedora package

      maintainers to ask upstreams to relicense such files.

    The README mention that

      All code is triple-licensed under the CC0, the OpenSSL Licence, or the Apache Public License 2.0, at your choosing.

    This comment is 7 years old so in that time reference to OpenSSL license means Apache-1.0.

    

    This is part of https://fedoraproject.org/wiki/Changes/SPDX_Licenses_Phase_2