Update fake, paket and tests to dotnet8

[gdziadkiewicz]

This pull request modernizes the build system and updates dependencies to use .NET 8, while removing legacy build scripts and tools. It migrates the build process from FAKE scripts to a .NET SDK-style project, updates solution and project files accordingly, and ensures all tooling and test projects are compatible with .NET 8. Additionally, it updates test frameworks and cleans up obsolete references.

Build system modernization:

• Migrated the build process from a FAKE script (build.fsx) to a .NET SDK-style executable project (build/build.fsproj, build/build.fs), updating all related scripts and CI workflows to use dotnet run --project build/build.fsproj instead of dotnet fake run build.fsx. [1] [2] [3] [4] [5] [6] [7] [8] [9]

• Removed legacy build artifacts and script references from the solution file (FSharpx.Collections.sln), and added the new build project. [1] [2] [3] [4]

Dependency and framework upgrades:

• Updated all relevant projects, dependencies, and tooling to target .NET 8 (net8.0), including global.json, test projects, and documentation projects. [1] [2] [3] [4] [5] [6] .config/dotnet-tools.json [7]

Test infrastructure updates:

• Switched Fable test projects from Fable.Jester and Fable.FastCheck to Fable.Mocha, updating test code accordingly and adding a program entry point. [1] [2] [3] [4]

Version and metadata updates:

• Bumped assembly versions from 3.0.1 to 3.1.0 in both FSharpx.Collections and FSharpx.Collections.Experimental. [1] [2] [3] [4]

Minor code and compatibility fixes:

• Fixed a pattern matching bug in TaggedCollections.fs for correct set comparison.