ci: update k3s test versions
- We want to support 1.25.
- 1.20 and 1.21 are deprecated
See https://docs.gitlab.com/ee/user/clusters/agent/#supported-cluster-versions for supported versions.
Part of gitlab-org/gitlab#371687 (closed).
See https://docs.gitlab.com/ee/user/clusters/agent/#supported-cluster-versions for supported versions.
Part of gitlab-org/gitlab#371687 (closed).