[Juju 3.6] Pick newest base among the allowed

[dimaqq]

Description

Context:

what is deployed when someone types “juju deploy ubuntu”?

what charm version?

on what base?

Summary:

python-libjuju calls ResolveCharms

Juju 3.0 ~ 3.5 returns ch:amd64/jammy/ubuntu-25

Juju 3.6 however returns ch:amd64/ubuntu-25

Meanwhile ch:amd64/ubuntu-25 appear to be everything on charmhub, from jammy to vivid.

Current code picks vivid which juju controller rejects.

Land a change that picks the newest instead of the oldest.

Urgency

Blocker for our release

Python-libjuju version

any

Juju version

3.6

Reproduce / Test

model.deploy("ubuntu")