KEP-2170: Rename TrainingRuntimeRef to RuntimeRef API

[andreyvelich]

Related: #2170

Since this PR introduced ability to register custom runtimes: #2248, we decided to rename the API from TrainingRuntimeRef to RuntimeRef in TrainJob.

In the future, we are planning to support custom user runtimes (e.g. Slurm, Flux, etc.) that might use their own CRDs to orchestrate K8s resources.
The RuntimeRef API gives us more flexibility in the future and not confuse users that only TrainingRuntime and ClusterTrainingRuntime CRDs can be used.

/assign @kubeflow/wg-training-leads
/cc @ahg-g @kannon92 @danielvegamyhre @vsoch

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In response to this:

Related: #2170

Since this PR introduced ability to register custom runtimes: #2248, we decided to rename the API from TrainingRuntimeRef to RuntimeRef in TrainJob.

In the future, we are planning to support custom user runtimes (e.g. Slurm, Flux, etc.) that might use their own CRDs to orchestrate K8s resources.
The RuntimeRef API gives us more flexibility in the future and not confuse users that only TrainingRuntime and ClusterTrainingRuntime CRDs can be used.

/assign @kubeflow/wg-training-leads
/cc @ahg-g @kannon92 @danielvegamyhre @vsoch

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