[prometheus-thanos] upgrade to v0.18.0

[hmlkao]

What this PR does / why we need it:

Upgrade Thanos to v0.18.0

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])

[hmlkao]

@monotek Any requirements from my side before merge? I just rebased to upstream master. ;)