Add solution explorer goldbar when targeting out-of-support

[timheuer]

Summary

Today we emit out-of-support warnings to the build outputs when targeting out-of-support framework versions:

Severity	Code	Description	Project	File	Line	Suppression State
Warning	NETSDK1138	The target framework 'net5.0' is out of support and will not receive security updates in the future. Please refer to https://aka.ms/dotnet-core-support for more information about the support policy.	ConsoleApp144	C:\Program Files\dotnet\sdk\8.0.100\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.EolTargetFrameworks.targets	32	

We do not put this more boldly in the IDE experience until then. We should consider adding a solution-explorer goldbar to show the same warning:

User Impact

More direct feedback about targeting out of support frameworks.

[adamint]

@mckennabarlow thoughts? I would be hesistant to make a decision like this without pm input

[mckennabarlow]

+1 to @timheuer's concerns about the visibility of this out-of-support warning. While I am generally mindful of "gold-bar-fatigue," this seems like an important warning to surface more boldly in the IDE.