Update Icon classes

[mtbayley]

Description

Closes #5737

• Update Icon classes to const string
• Update to FontAwesome 6.6.0

How Has This Been Tested?

Tested using Icon page in BasicTestApp

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality not to work as expected)

Checklist:

• The PR is submitted to the correct branch (master for dev, rel-1.x for maintenance).
• My code follows the code style of this project.
• I've added relevant tests.

[github-actions]

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[mtbayley]

I have read the CLA Document and I hereby sign the CLA

[mtbayley]

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