Add .NET 8-10 targets

[paulirwin]

This PR adds the net8.0, net9.0, and net10.0 targets per the discussion/roadmap in #44. These new targets are not signed with a strong name key; only the netstandard2.0 target will be signed.

Fixes#50

[Copilot]

Pull Request Overview

This PR adds support for .NET 8, 9, and 10 as additional target frameworks alongside the existing netstandard2.0 target. The key architectural change is that only the netstandard2.0 target will be signed with a strong name key, while the newer .NET targets remain unsigned.

• Adds net8.0, net9.0, and net10.0 as new target frameworks
• Conditionally applies strong-name signing only to netstandard2.0 target
• Updates project structure to use newer .slnx solution format and removes legacy Directory.Build.props files

Reviewed Changes

Copilot reviewed 9 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/F23.StringSimilarity/F23.StringSimilarity.csproj	Adds multiple target frameworks, conditional signing for netstandard2.0, and updates package versions
src/F23.StringSimilarity/Properties/AssemblyInfo.cs	Conditionally applies InternalsVisibleTo with public key only for signed assemblies
test/F23.StringSimilarity.Tests/F23.StringSimilarity.Tests.csproj	Adds conditional signing configuration for net481 target
benchmarks/F23.StringSimilarity.Benchmarks/F23.StringSimilarity.Benchmarks.csproj	Updates target framework to net10.0 and BenchmarkDotNet package version
F23.StringSimilarity.slnx	Adds new .slnx solution file format
F23.StringSimilarity.sln	Removes legacy .sln file
Directory.Build.props	Removes global signing configuration
src/Directory.Build.props	Removes source-level build props file
test/Directory.Build.props	Removes test-level build props file

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}