Update the upload download actions to v4

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Update the GitHub actions for upload and download artifacts to v4.

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Testing

Describe how you've tested these changes.

https://github.com/firebase/firebase-unity-sdk/actions/runs/11734287899
https://github.com/firebase/firebase-unity-sdk/actions/runs/11735191421

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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