[prometheus-thanos] update thanos to 0.12.2

[nvanheuverzwijn]

What this PR does / why we need it:

Update prometheus-thanos from 0.12.0 to 0.12.2. This version of thanos include a bug fix that would prevent thanos from deleting partial block and would cause a lot of spam.

See https://github.com/thanos-io/thanos/blob/v0.13.0/CHANGELOG.md#v0122---20200430

Special notes for your reviewer:

0.12.1 is skipped because it does not fix this bug entirely while 0.12.2 does.

Checklist

• [x] DCO signed
• [x] Chart Version bumped (if the pr is an update to an existing chart)
• [x] Title of the PR starts with chart name (e.g. [fluentd-elasticsearch])

[nvanheuverzwijn]

@rpahli @axdotl Care for review ? I will bring 0.13.0 whenever this get merged. Thanks!