license: switch from BSD 4-clause to 3-clause

license: switch from BSD 4-clause to 3-clause

Remove the advertising clause (clause 3) and renumber the
non-endorsement clause from 4 to 3 in all files containing the
BSD 4-clause license, following the notice issued by UC Berkeley
(quoted below).

Additional changes:
- Add SPDX-License-Identifier: BSD-3-Clause to each affected file.
- Remove the BSD-4-Clause-UC entry from README.licensing.
- Delete Documentation/licenses/COPYING.BSD-4-Clause-UC.

Reference:
https://ipira.berkeley.edu/sites/default/files/amendment_of_4-clause_bsd_software_license.pdf

Relevant text:

    January 31, 2012

    To all licensees, and distributors of software officially
    licensed under a UC Berkeley version of the BSD license:

    As you may know, certain software officially licensed by UC
    Berkeley using the original 4-clause BSD license required that
    further distributions of products containing all or portions of
    the software acknowledge within their advertising materials that
    such products contain software developed by UC Berkeley and its
    contributors.

    Specifically, the provision reads:

        "3. All advertising materials mentioning features or use of
        this software must display the following acknowledgement:
        This product includes software developed by the University
        of California, Berkeley and its contributors."

    Effective immediately, licensees and distributors are no longer
    required to include the acknowledgement within advertising
    materials. Accordingly, the advertising clause 3 of the original
    4-clause BSD license for any and all software officially licensed
    by UC Berkeley, is hereby deleted in its entirety.

    Michael Katz
    Executive Director
    Intellectual Property and Industry Research Alliances
    University of California, Berkeley