testing/thanos: upgrade to 0.31.0
https://github.com/thanos-io/thanos/blob/v0.31.0/CHANGELOG.md
quite a lot of changes, seems to work at first glance though
https://github.com/thanos-io/thanos/blob/v0.31.0/CHANGELOG.md
quite a lot of changes, seems to work at first glance though