Update target framework to .NET 8, and Umbraco dependency to 14

[mdeslauriers-sigmund]

• Update target framework to .NET 8
• Update Umbraco dependency to 14

[ZioTino]

Hi @mdeslauriers-sigmund and thank you for your contribution. Can I ask though why you're requesting this update? You should be able to use the package already with newer versions of .NET and Umbraco, or am I missing something?

[mdeslauriers-sigmund]

Sorry for not explaining in the commit message

For Umbraco 14+, a project cannot build when referencing the previous version :

You get the following errors :

The call is ambiguous between the following methods or properties: 
'Umbraco.Extensions.UmbracoBuilderExtensions.AddBackOffice(Umbraco.Cms.Core.DependencyInjection.IUmbracoBuilder, 
System.Action<Microsoft.Extensions.DependencyInjection.IMvcBuilder>?)' and 
'Umbraco.Extensions.UmbracoBuilderExtensions.AddBackOffice(Umbraco.Cms.Core.DependencyInjection.IUmbracoBuilder, 
System.Action<Microsoft.Extensions.DependencyInjection.IMvcBuilder>)'

In Program.cs you have the following code :

builder.CreateUmbracoBuilder()
    .AddBackOffice()
    .AddWebsite()
    .AddDeliveryApi()
    .AddComposers()
    .Build();

The extension method .AddBackOffice(), seems to have changed signature between v9 and v14.