Support .NET 8, bump System.Text.Json from 6.0.0 to non-vulnerable 8.0.5

[inikulshin]

I extracted non-API changes from #349 to separate PR.

[inikulshin]

I tried to run integration tests on Windows, bug gave up:

docker-library/rabbitmq#168 (comment)

[inikulshin]

I also downgraded Microsoft.CodeAnalysis.CSharp to 4.8.0:

@Pliner you removed CodeQL workflow 2 years ago (6af477f), but it's still active:

https://github.com/EasyNetQ/EasyNetQ.Management.Client/actions/workflows/github-code-scanning/codeql

Was it intended?

• For this PR (and last few months) CodeQL workflow fails because it uses old MSBuild:

  [2025-02-20 20:37:30] [build-stdout] Running dotnet clean /home/runner/work/EasyNetQ.Management.Client/EasyNetQ.Management.Client/Source/EasyNetQ.Management.Client.sln
  [2025-02-20 20:37:30] [build-stdout] MSBuild version 17.8.5+b5265ef37 for .NET

Since #323, MSBuild version must be >= 17.9, according to

https://github.com/dotnet/roslyn/blob/main/docs/wiki/NuGet-packages.md#versioning

@zidad FYI

[lukebakken]

I tried to run integration tests on Windows

Install Erlang and RabbitMQ via their .exe installers in the same manner as RabbitMQ.Client:

• https://github.com/rabbitmq/rabbitmq-dotnet-client/blob/main/.ci/windows/gha-setup.ps1
• https://github.com/rabbitmq/rabbitmq-dotnet-client/blob/main/.github/workflows/build-test.yaml#L54-L60