Change the default runner image to grafana/k6:latest

[yorugac]

Feature Description

k6-operator currently uses a public image hosted at our ghcr as a default image for runners.
This image is built from the Dockerfile on each release of k6-operator. Under the hood, runner image is the grafana/k6:latest with the wrapper that makes it usable in Envoy setup by default.

This approach has a drawback: default version of k6 used by k6-operator depends on the time of release:
https://github.com/grafana/k6-operator/blob/main/docs/versioning.md
Worse, for k6-operator to use the latest version of k6 by default (i.e. without utilizing .spec.runner.image in TestRun), the operator must be released after corresponding k6 release.

Suggested Solution (optional)

We can switch to use grafana/k6:latest image as the default in the k6-operator. The latest k6 historically points only to the latest release and can be considered stable. This way, k6-operator will always use the latest stable version of k6 by default, with all features included.

However, this will break current default compatibility with Istio: users who rely on it will have to start passing custom runner images in TestRun spec.

If we go this route, there will also be a related question of whether we should continue to build and host runner images. We can continue to build them as now, on k6-operator release, assuming Istio users will use them in .spec.runner.image of TestRun.

Community feedback will be appreciated!

Already existing or connected issues / PRs (optional)

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