Support C# 12 by installing .NET 8 SDK in the Ubuntu pipeline

[Bartleby2718]

Motivation

The current Ubuntu pipeline doesn't have .NET 8 SDK installed.

Background

According to https://www.appveyor.com/docs/linux-images-software/, we're currently using Ubuntu 18.04 (image name: Ubuntu) in the CI pipeline.

However, .NET 8 SDK is not installed in any of default Ubuntu images according to the same page. Until https://github.com/appveyor/ci/issues/3908 is complete, we'll need to install .NET SDK manually.

However, you cannot install .NET 8 on Ubuntu 18.04 per https://learn.microsoft.com/en-us/dotnet/core/install/linux-ubuntu-1804.

Changes in the PR

Updated the image to the latest Ubuntu version (that's supported in AppVeyor) 22.04 and manually installing .NET 8 SDK per https://learn.microsoft.com/en-us/dotnet/core/install/linux-ubuntu-2204.

Testing

• I included C# 12 changes like primary constructors and collection expressions, which come with .NET 8 SDK, so this should be a good litmus test.
• The Windows build is failing due to #110.

[Bartleby2718]

@madelson No transient failures this time!