[dotnet] Align paket target framework with supported versions (#15212)

Author: nvborisenko

dotnet/paket.dependencies

@@ -1,5 +1,5 @@
 group nuget
-framework: net7.0,netstandard2.0
+framework: net8.0,netstandard2.0
 source https://api.nuget.org/v3/index.json
 
 nuget BenderProxy 1.0.0