adopters: add Elastic

[v1v]

Add Elastic as adopter.

We are transitioning from adhoc to this model, so there will be more repositories with the automation.

cc @kuisathaverat , you were the pioneer to use it in some of the projects at Elastic. 🙇

[ghost]

👇 Click on the image for a new way to code review

• Make big changes easier — review code in small groups of related files

• Know where to start — see the whole change at a glance

• Take a code tour — explore the change with an interactive tour

• Make comments and review — all fully sync’ed with github

  Try it now!

Legend