[WIP] Rename ephemeral_storage to disk for simplicity

[JiangJiaWei1103]

Tracking issue

flyteorg/flyte#6142

Why are the changes needed?

The attribute ephemeral_storage of Resources (i.e., K8s pod resources) is too complicated and should be simplfied.

What changes were proposed in this pull request?

At this stage, we propose supporting both disk and ephemeral_storage with the following behavior:

1. ephemeral_storage will remain supported as an alias for backward compatibility and developer-facing code
2. Users are encouraged to adopt disk for future configurations

The behavior for different configurations is summarized as follows:

1. Both disk and ephemeral_storage are specified: Raise an error to avoid ambiguity
2. Only ephemeral_storage is specified: Issue a warning about future deprecation but continue to use it as is
3. Only disk is specified: Automatically set ephemeral_storage to the value of disk

How was this patch tested?

Setup process

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Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

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[flyte-bot]

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