Updated the project to target multiple frameworks (`net462`,
`net480`, and `net8.0`) by replacing `<TargetFramework>` with
`<TargetFrameworks>`. Added `<LangVersion>` property set to
`latest` to enable the use of the latest C# language features.
Adjusted `<Nullable>` property formatting for consistency.
Updated project and solution files to rename and restructure
`DocumentOperator` to `DocumentService`. Modified project
references in `DocumentService.API.csproj`, `DocumentService.Application.csproj`,
and `DocumentService.Infrastructure.csproj` to reflect the new
naming convention. Updated `DocumentService.sln` to remove
old project references, add new ones, and adjust configuration
and nested project mappings accordingly. This change aligns
the project structure with the new naming convention.
