Retarget library, sample, and CI to .NET 10

[Copilot]

This updates the repository to target .NET 10 across the shipped library, test project, sample app, and GitHub Actions workflows. It also updates the NuGet package metadata to carry the 2021-2026 copyright range and aligns the README with the repository’s current target.

• Framework targeting

  • Retargets the main package, test project, and sample app from net9.0 to net10.0.

• CI/CD

  • Updates actions/setup-dotnet usage in CI and release workflows from 9.0.x to 10.0.x so hosted builds match the project target.

• Package metadata

  • Updates the package copyright to 2021-2026 to preserve the original publication year while reflecting the 2026 update.

• Sample app wiring

  • Replaces the sample’s hard-coded DLL reference with a project reference so it builds from source in any configuration.

• Documentation

  • Clarifies in the README that the current repository sources target .NET 10.

<PropertyGroup>
  <TargetFramework>net10.0</TargetFramework>
</PropertyGroup>

<ItemGroup>
  <ProjectReference Include="..\..\src\NetCore.Utilities.Spreadsheet\NetCore.Utilities.Spreadsheet.csproj" />
</ItemGroup>

• Fixes Update Project to target .NET 10 #151