Update icon to junit 6 icon

[hanszt]

Update Intellij icon to JUnit 6 icon

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I hereby agree to the terms of the JUnit Contributor License Agreement.

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[sormuras]

Soon ™️ ... it will be superseded by

• Decide for and adopt new project logo #4576

anyway. So, let's skip this step going from "5" over "6" to "numberless".

[marcphilipp]

I don't see any harm to update to the "temporary" icon in the meantime.