Publish loadimpact/k6 Docker image with a deprecation warning

[imiric]

Given that Docker Hub is sunsetting Free Team organizations effective April 14th, 2023, we need to deprecate our loadimpact/k6 image, which is on that tier.

Our plan is to continue to support it for at most a year from now, but we need to add a deprecation warning to it pointing users to the grafana/k6 image instead.

This can be done with an entrypoint script that wraps k6, and prints a warning before running. A few months before the end of that year period, tentatively towards the end of 2023, the entrypoint script will additionally start returning a non-zero exit code as well.

[imiric]

This is no longer a high priority since Docker backtracked their decision 🙄, but we should still do this regardless, since we're not Load Impact anymore.

[andrewslotin]

Closed by #3121