Deprecate MXJob from Training Operator

[andreyvelich]

Unfortunately, Apache MXNet project has been archived: https://attic.apache.org/projects/mxnet.html
Original GitHub repo: https://github.com/apache/mxnet.

This issue is to track plan for Training Operator to remove MXJob support.
We are going to deprecate MXJob with the two stages:

• In Training Operator 1.8 release we will notify users that MXJob is deprecated and it will be removed in the next release version.

  • We need to update the Kubeflow website
  • We need to add a deprecation notice on the MXJob APIs.

• In Training Operator 1.9 release we will remove support for MXJob.

@terrytangyuan @tenzen-y @kubeflow/wg-training-leads Any other places where we should add deprecation notice ?

[tenzen-y]

@kubeflow/wg-training-leads If we can pin this issue, it would be great.