Reduces revised-versions section to MPL-style built-in or-later.

Reduces revised-versions section to MPL-style built-in or-later.

The revised-versions section of the GNU GPL, particularly GNU GPLv3,
is necessarily complicated by the FSF's policy decision to leave
authorization to use later versions to the licensor's discretion.

I am not settled on built-in "or later" as a desired policy. I respect
the concerns of GNU GPL licensors who take care both to specify a
version and to deny downstream authorization to use later versions. On
the other hand, most other copyleft licenses have a built-in "or
later" feature. Perhaps "or later" should be a default, but some
authorization for licensor opt-out should be formalized.  But already
at least one person who is following GPL.next development has
criticized such a proposal.

For the time being I am proceeding with built-in or-later, modeled
closely on the MPL 2.0 approach. This allows significant
simplification of the revised-versions section.

A different feature of the GNU GPL revised-versions section is its
effort to deal with the possibility of uses of the GNU GPL without
specification of a version. This is quite common in the real world,
but it is of course an academic issue for GPL.next.