Update OrderService.csproj from .NET Core 2.1 to LTS (or current) version

[github-actions]

The OrderService.csproj project file targets a .NET version which is no longer supported. This is an auto-generated issue, detailed and discussed in dotnet/docs#22271.

See line 5 in workshops/dockerton-2018/kubernetes-workshop-orders/src/OrderService/OrderService.csproj.

Target version	End of life	Release notes	Nearest LTS TFM version
netcoreapp2.1	August, 21 2021	netcoreapp2.1 release notes	netcoreapp3.1

Consider upgrading the project to either the current release, or the nearest LTS TFM version.

If this project is intentionally targeting an unsupported version, you can optionally configure to ignore this automated issue. Create a file at the root of the repository, named dotnet-versionsweeper.json and add an ignore entry following the globbing patterns detailed here.

{
    "ignore": [
        "**/workshops/dockerton-2018/kubernetes-workshop-orders/src/OrderService/OrderService.csproj"
    ]
}