Refactor to use Thanos v0.20.1

[gmfrasca]

We would like to have the thanos dependency's version incremented v0.20.1 (and conflicts stemming from that resolved). This version of the thanos code has new features, one of which that we're particularly interested in is the ability to use self-signed certificates for the storeapi grpc connection.

Unfortunately it's a relatively large jump, (from v0.13.1), and it looks like a simple version bump in go.mod throws a bunch of dependency conflicts or build errors, so it appears to be non-trivial/a bit more complicated than that.

[anishasthana]

When we took a stab at upgrading it, we ended up with the following https://github.com/anishasthana/obslytics/tree/update_dependencies. We have not been able to get around one of the dependency failures. Pinning gnostic to v0.4.0 resulted in another import failing as one of the other modules requires v0.5.1 or higher.

> go mod tidy
go: finding module for package github.com/googleapis/gnostic/openapiv2
github.com/thanos-community/obslytics/pkg/series/promread imports
        github.com/prometheus/prometheus/storage/remote imports
        github.com/prometheus/prometheus/config tested by
        github.com/prometheus/prometheus/config.test imports
        github.com/prometheus/prometheus/discovery/kubernetes tested by
        github.com/prometheus/prometheus/discovery/kubernetes.test imports
        k8s.io/client-go/kubernetes/fake imports
        k8s.io/client-go/testing imports
        k8s.io/apimachinery/pkg/util/strategicpatch tested by
        k8s.io/apimachinery/pkg/util/strategicpatch.test imports
        k8s.io/apimachinery/pkg/util/strategicpatch/testing imports
        github.com/googleapis/gnostic/openapiv2: module github.com/googleapis/gnostic@latest found (v0.5.5, replaced by github.com/googleapis/gnostic@v0.4.0), but does not contain package github.com/googleapis/gnostic/openapiv2