USWDS-Site - Update Adobe XD support status #2637

Describe the bug

Adobe XD is now officially end-of-life and no longer supported, so we should announce / note we will no longer be updating or supporting our Adobe XD design asset.

As a first step, I updated the ReadMe of uswds-for-designers to add: "Unfortunately, Adobe XD is no longer supported by Adobe, so we won't be making any further updates to this file."

It'll definitely need to be updated on Getting started for designers, but I suspect other places too.

As a draft of a checklist:

• identify what needs to be updated
• write updates
• publish updates

Steps to reproduce the bug

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Expected Behavior

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Screenshots

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System setup

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Additional context

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Code of Conduct

• I agree to follow this project's Code of Conduct.
• I checked the current issues for duplicate bug reports.