V15: Update to dotnet 9

[Zeegaan]

Notes

• Had to rename TrimStart & TrimEnd, as they are now implemented by Microsoft, and the behavior differs from ours.
• Updates dotnet verison to dotnet9
• Updates nuget packages to their respective dotnet 9 version
• Had to update Umbraco.Code to 2.2 aswell, as it used the Microsoft.CodeAnalysis package, and thus we couldn't update that.

How to test

Ensure both templates & source code work with dotnet 9

• Download dotnet 9 preview here: https://dotnet.microsoft.com/en-us/download/dotnet/9.0
• In the root of the project, run dotnet pack -o nuget
• Go to the nuget folder cd nuget
• Copy the path of that folder, and then run dotnet nuget add source [PATH TO NUGET FOLDER HERE]
• Install the templates, take note of the version of the templates nupkg in the nuget folder dotnet new install Umbraco.Templates::[version of templates here]
• Create a new umbraco project with the new template dotnet new umbraco -n "MyTestProject"
• Run the project and assert it works 👍

[elit0451]

Can you have a look at the pipelines and update the <TargetFramework> to net9.0 in the csproj's of our dotnet new templates? And the .template.config/template.json has also mentions of net8.0

[ronaldbarendse]

I've had a quick look and added my comments/suggestions 😄

Seems like we can actually do more cleaning up of the StringExtensions class, as there's a lot of unused methods or ones that are very specific to our own codebase and could therefore be changed to internal...

[Zeegaan]

@ronaldbarendse Cleanup sounds great, but is out of scope for this PR IMO, we can always make a separate one with cleanups 👍

[elit0451]

Looks good and tests out good 💪 Merging