Add v0.18.0 CHANGELOG

[dgrisonnet]

Since we are a bit late for the v0.18.0 release, my plan is to only push the CHANGELOG to master and update the manifests in the release branch directly. I would go with the following process for the release branch:

• Cut the release-branch based on e00c4f2 with the following changes https://github.com/thanos-io/kube-thanos/compare/release-0.17...e00c4f2. This include all the changes made before the release of thanos v0.18.0 the 1st of February.
• Cherry-pick changes made to the changelog. Should we also keep the unreleased changes?
• Update manifests
• Tag

Let me know if this sounds good to you.

/cc @kakkoyun