[MessageBar] Update way animation is applied to the MessageBar

[yspanj]

Pull Request

📖 Description

Updated the was the animation is applied to the MessageBar component.
As discussed in #2716

✅ Checklist

General

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• I have added a new component
• I have added Unit Tests for my new component
• I have modified an existing component
• I have validated the Unit Tests for an existing component

⏭ Next Steps

[vnbaaij]

Do we have a test for the 'null'-variant as well? (working from my phone)

[yspanj]

Do we have a test for the 'null'-variant as well? (working from my phone)

That should be covered by the FluentMessageBar_DisableFadeIn() test case added in the previous pull request.
But since the animation attribute isn't rendered the test didn't need to be updated for this change.