Update to .NET 8.0 GA

[brminnick]

Description of Change

This PR updates global.json, azurepipelines.yml and DirectoryBuild.props in anticipation of the upcoming .NET 8.0 GA release;

Additional information

The CI build is expected to fail until the GA release of .NET 8.0

[brminnick]

Hey @JoonghyunCho! Can you help us fix the Tizen build error?

C:\hostedtoolcache\windows\dotnet\packs\Samsung.Tizen.Sdk\7.0.128\targets\Samsung.Tizen.Sdk.Packaging.targets(161,5): error : The api-version specified in tizen-manifest file is 7. [D:\a\1\s\samples\CommunityToolkit.Maui.Sample\CommunityToolkit.Maui.Sample.csproj::TargetFramework=net8.0-tizen]
C:\hostedtoolcache\windows\dotnet\packs\Samsung.Tizen.Sdk\7.0.128\targets\Samsung.Tizen.Sdk.Packaging.targets(161,5): error : Current target framework net8.0-tizen is not supported in this api-version. [D:\a\1\s\samples\CommunityToolkit.Maui.Sample\CommunityToolkit.Maui.Sample.csproj::TargetFramework=net8.0-tizen]

https://dev.azure.com/dotnet/CommunityToolkit/_build/results?buildId=100402&view=logs&j=67da6a11-18ef-5f70-9b43-fae7e6c83e18&t=f71eeafc-b93b-5d4c-447b-0401a1900176&l=860

[JoonghyunCho]

Hey @JoonghyunCho! Can you help us fix the Tizen build error?

C:\hostedtoolcache\windows\dotnet\packs\Samsung.Tizen.Sdk\7.0.128\targets\Samsung.Tizen.Sdk.Packaging.targets(161,5): error : The api-version specified in tizen-manifest file is 7. [D:\a\1\s\samples\CommunityToolkit.Maui.Sample\CommunityToolkit.Maui.Sample.csproj::TargetFramework=net8.0-tizen]
C:\hostedtoolcache\windows\dotnet\packs\Samsung.Tizen.Sdk\7.0.128\targets\Samsung.Tizen.Sdk.Packaging.targets(161,5): error : Current target framework net8.0-tizen is not supported in this api-version. [D:\a\1\s\samples\CommunityToolkit.Maui.Sample\CommunityToolkit.Maui.Sample.csproj::TargetFramework=net8.0-tizen]

https://dev.azure.com/dotnet/CommunityToolkit/_build/results?buildId=100402&view=logs&j=67da6a11-18ef-5f70-9b43-fae7e6c83e18&t=f71eeafc-b93b-5d4c-447b-0401a1900176&l=860

I should have fixed it in advance... finger crossed for the green build now!