Fixing additional errors presented in Unity package validations

[AMollis]

Overview

Fixing additional errors presented in Unity package validations, and download grading validation package due to publishing bugs.

Unity's package validation requires uncompressed image assets. So, I've converted some remaining JPGs to PNGs.

The com.unity.asset-store-validation version 3.0.0 is not compatible with Unity publishing system. As such, we need to downgrade to version 0.2.1

Verification

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As a reviewer, it is possible to check out this change locally by using the following
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git fetch origin pull/{PR_ID}/head:name_of_local_branch

git checkout name_of_local_branch

[Cameron-Micka]

Tested locally, looks great! Thank you.