Update to .NET 9

[TheCodeTraveler]

Description of Change

This PR updates CommunityToolkit.Maui.Markup for .NET 9

PR Checklist

• [x] Has tests (if omitted, state reason in description)
• [x] Has samples (if omitted, state reason in description)
• [x] Rebased on top of main at time of PR
• [x] Changes adhere to coding standard
• [x] Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pull/480

Additional information