licensing: add MPL-2.0+ as equivalent to MPL-2.0
Permission to use later versions is included in the license, so the
"-or-later" part does not change anything.
> 10.2. Effect of New Versions
> You may distribute the Covered Software under the terms of the version
> of the License under which You originally received the Covered
> Software, or under the terms of any subsequent version published by
> the license steward.