Update library dependencies

[Misha-133]

Some dependencies are outdated or even unsupported, so this PR bumps deps of the lib and samples to more recent versions

Changes

• bump Newtonsoft.Json 13.0.1 => 13.0.3

• bump Microsoft.CodeAnalysis 3.11.0 => 4.9.2

• bump IDisposableAnalyzers 3.4.15 => 4.0.7

  • apply analyzer suggestion (make internal class sealed)

• bump System.Interactive.Async 5.0.0 => 6.0.1

• bump System.Collections.Immutable 5.0.0 => 6.0.0

• bump Microsoft.NET.Test.Sdk 16.11.0 => 17.9.0

• bump Microsoft.Extensions.DependencyInjection.Abstractions 5.0.0 => 6.0.0

• bump xunit 2.4.1 => 2.6.2

• bump xunit.runner.visualstudio 2.4.3 => 2.5.4

• bump FluentAssertions 6.7.0 => 6.12.0

  • add some syntax sugar to tests

• bump Discord.Net to 3.13.1 in samples

• bump samples to net8.0