replace netcoreapp5, netcoreapp6 with net5 and net6

netcoreapp was [replaced](https://learn.microsoft.com/en-us/dotnet/standard/frameworks#supported-target-frameworks) with net since v5. some projects are using `netcoreapp5.0` and `netcoreapp6.0`, which should be replaced with net5.0 and net6.0 because the fallback logic might be removed in the future from sdk.