feat(csharp): Update CI workflow inside C# SDK generator

[Swimburger]

Description

Simplify and optimize the CI workflow & update the GH actions dependencies & use .NET 9 as the runtime host.

Changes Made

• Merge three jobs into a single job to avoid re-do'ing the same work multiple times (installing .NET, restoring packages, building project)
• Make sure the versions of .NET that we're compiling are installed
• Also, install the .NET 9 SDK as the MSBuild and .NET CLI in this version have improvements even when compiling for older TFMs.
• Break up .NET build into separate steps, each depending on the previous step
  1. Restore dependencies
  2. Build solution (without restore)
  3. Test solution (without build & without restore)
  4. Pack solution (without build & without restore), regardless of whether tests pass (as it worked previously)
• Reducing irrelevant log output by setting DOTNET_SKIP_FIRST_TIME_EXPERIENCE and DOTNET_NOLOGO env variables
• Update GitHub actions (actions/checkout & actions/setup-dotnet)

Since now it's a single job, the UI will look different.
If we really want the UI to show three different jobs, we can revert to three jobs, but it'll slow down the pipeline significantly without any real benefit AFAIK.

Testing

• Manual testing completed

[github-actions]

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