unsupport non-standard TFM netXY vs. netX.Y

[kasperk81]

0.01% of the projects on github have <TargetFramework>net80 https://github.com/search?q=%22%3Ctargetframework%3Enet80%22&type=code vs net8.0 https://github.com/search?q=%22%3Ctargetframework%3Enet8.0%22&type=code

XY is undocumented format https://learn.microsoft.com/dotnet/standard/frameworks#supported-target-frameworks and double digit tfm will make life harder for third party tools to parse the value (dotnet/runtime#106599 (comment)).

sdk should just error on netXY netcoreappXY netcoreappX.Y for v >= 5.

[marcpopMSFT]

@baronfel do we have an issue tracking sdk buildchecks?

[baronfel]

Just the msbuild-analyzer-candidate label that's been around for a bit.