fix: Replace border with box-shadow to avoid layout shifts

[jungwoo3490]

Reopen kubeflow/kubeflow#7659

There was an issue where, upon pressing the "Create Notebook" button and then selecting the notebook environment, the layout of the remaining buttons shifted due to the border.

I consider this unintended shift negatively impacts the user experience.

So I replaced border with box-shadow to avoid unintended layout shifts and improve user experience.

AS-IS

2024-11-12.9.06.44.mov

TO-BE

2024-11-12.9.56.42.mov

[google-oss-prow]

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