I cloned your project and reproduced the problem. I have checked the dependnecies of the nuget package and found that the CommunityToolkit.Maui 9.1.1 didn't have the dpendency for .net 9.0.
And the CommunityToolkit.Maui 10.0.0 and CommunityToolkit.Maui 11.0.0 both have dependency for .net 9.0.
When I used the two versions of the CommunityToolkit.Maui nuget package and rebuild the project, the warning in your example disappeared.