Updates for .NET 8 - release-8.1

[particularbot]

• Set CI workflow dotnet-version to 8.0.x
• Set release workflow dotnet-version to 8.0.x
• Set test project target frameworks (not including .NET Framework) to net8.0
• Update GitHub Actions versions:
  • actions/checkout to v4.1.2
  • actions/setup-dotnet to v4.0.0
  • actions/upload-artifact to v4.3.1
  • Particular/run-tests-action to v1.7.0
  • Particular/push-octopus-package-action to v2.0.0
  • azure/login to v2.0.0
• Update build tool NuGet package versions in test projects:
  • GitHubActionsTestLogger to 2.3.3
  • Microsoft.NET.Test.Sdk to 17.9.0
  • NUnit to 3.14.0
  • NUnit3TestAdapter to 4.5.0