.github: drop support for 1.20 on main

According to our policy, main branch of kube-prometheus should support
the 2 latest versions of Kubernetes. These changes update the tests and
the compatibility matrix to reflect that.

Signed-off-by: Damien Grisonnet <dgrisonn@redhat.com>

Author: dgrisonnet

.github/workflows/ci.yaml

@@ -52,7 +52,6 @@ jobs:
     strategy:
       matrix:
         kind-image:
-          - 'kindest/node:v1.20.7'
           - 'kindest/node:v1.21.1'
           - 'kindest/node:v1.22.0'
     steps:

README.md

@@ -112,7 +112,7 @@ The following versions are supported and work as we test against these versions
 | [`release-0.6`](https://github.com/prometheus-operator/kube-prometheus/tree/release-0.6) | ✗               | ✔               | ✗               | ✗               | ✗               |
 | [`release-0.7`](https://github.com/prometheus-operator/kube-prometheus/tree/release-0.7) | ✗               | ✔               | ✔               | ✗               | ✗               |
 | [`release-0.8`](https://github.com/prometheus-operator/kube-prometheus/tree/release-0.8) | ✗               | ✗               | ✔               | ✔               | ✗               |
-| [`HEAD`](https://github.com/prometheus-operator/kube-prometheus/tree/main)               | ✗               | ✗               | ✔               | ✔               | ✔               |
+| [`HEAD`](https://github.com/prometheus-operator/kube-prometheus/tree/main)               | ✗               | ✗               | ✗               | ✔               | ✔               |
 
 ## Quickstart