Vagrant: update box

[clbarnes]

Ubuntu 18.04 is pretty old.

However, resizing the disk becomes a problem when using anything except bento/ubuntu-18.04 (e.g. bento/ubuntu-20.04, ubuntu/focal64, and even ubuntu/bionic64). The vagrant-disksize plugin resizes the "physical" disk but not the physical or logical volumes on top of it, as in this issue sprotheroe/vagrant-disksize#37 . The snippet in this comment sprotheroe/vagrant-disksize#37 (comment) works with the current box, but not with any of the others (see that thread).

Will hopefully resolve #2115 .