[CI] Move to elastic/ci buildkite implementation

[nickofthyme]

Currently we use our custom buildkite implementation that was modeled after the initial buildkite implementation in kibana. Since then the elastic/ci team have built out a canonical way of configuring buildkite pipelines across kibana.

Here are the starter docs

• https://docs.elastic.dev/ci/user-guide
• https://docs.elastic.dev/ci/getting-started-with-buildkite-at-elastic