Use "GNU GPL v3.0+" instead of "GNU GPL v3.0"

[xuhdev]

Free Software Foundation (the steward of the GNU GPL family licenses) encourages using gplvX+ instead of gplvX-only licensing. For example:

... This is why it is important to release under "GPL version N or later" -- so that we can fix such problems for them, even when they have many copyright holders.

https://www.fsf.org/licensing/zfs-and-linux .

See also the notes in https://spdx.org/licenses/GPL-3.0-or-later.html