[CI] Remove k8s 1.18.20 from testing matrix

[tbavelier]

What this PR does / why we need it:

• Removes Kubernetes v1.18.20 from testing matrix as the underlying action fails since ci: update Kube and test workflows #1486:

╰─❯ kind create cluster --config .github/kind_config.yaml --name chart-testing --wait 60s --image kindest/node:v1.18.20
Creating cluster "chart-testing" ...
 ✓ Ensuring node image (kindest/node:v1.18.20) 🖼
 ✓ Preparing nodes 📦 📦
 ✓ Writing configuration 📜
 ✗ Starting control-plane 🕹️
Deleted nodes: ["chart-testing-worker" "chart-testing-control-plane"]
ERROR: failed to create cluster: failed to init node with kubeadm: command "docker exec --privileged chart-testing-control-plane kubeadm init --config=/kind/kubeadm.conf --skip-token-print --v=6 --skip-phases=preflight" failed with error: exit status 1

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes #

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• Chart Version bumped
• Documentation has been updated with helm-docs (run: .github/helm-docs.sh)
• CHANGELOG.md has been updated
• Variables are documented in the README.md
• For Datadog Operator chart or value changes update the test baselines (run: make update-test-baselines)

[clamoriniere]

/merge

[dd-devflow]

🚂 MergeQueue: pull request added to the queue

The median merge time in main is 36m.

Use /merge -c to cancel this operation!

[davidor]

Is this just because 1.18.20 is no longer available in the kind version that we're using now or something else?

[tbavelier]

Is this just because 1.18.20 is no longer available in the kind version that we're using now or something else?

@clamoriniere will own the investigation/fix, but seems likely.

[dd-devflow]

🚂 MergeQueue: This merge request was already merged

This pull request was merged directly.