In order to migrate to running tests on https://clients-ci.elastic.co/,
we'll need to establish some conventional structure for controlling the
Jenkins jobs. The first convention is a build script that Jenkins will
execute, and the second will be a definition of the matrix axes values
to cover the version combinations that we'd like to test.
Having those two items defined within the repository means that only
minimal tweaking of the actual Jenkins job itself will be required in
the future.
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