Update BuildKite Gitlab Plugin

[CtfChan]

This https://buildkite.com/docs/pipelines/source-control/gitlab#commit-statuses saids commit statuses are not supported on Gitlab.

It looks like this is now supported: https://github.com/buildkite-plugins/gitlab-status-buildkite-plugin

[pzeballos]

I believe that note is because gitlab commit status are not natively supported in the product, so you need to use the plugin for this. But mentioning the plugin as a workaround there it's a good idea :)