Upgrade XGBoost to v3.0.5 and modernize build

[cdmwebs]

The core of the update is a major version upgrade of the underlying XGBoost C++ dependency, along with significant modernization of the build system and fixes for API compatibility.

Problem

The library was pinned to an older v2-era version of XGBoost. This resulted in build failures on up-to-date toolchains, including:

• Outdated CMake syntax rejected by modern CMake versions (3 -> 4).
• C code that was not compatible with modern Clang compilers on macOS.
• Elixir code that was incompatible with the new model configuration format in XGBoost v3, leading to MatchError exceptions.
• Deprecation warnings from recent Elixir versions and updated dependencies.

Solution

This PR addresses these issues with the following key changes:

• Upgraded Core Dependency: The XGBoost C++ library is now pinned to the modern stable version v3.0.5.
• Modernized Build System: The Makefile has been refactored to use a standard, incremental build process with CMake and Ninja. The old sed patch and macOS-specific LLVM workarounds have been removed.
• API and Compiler Fixes: The NIF C code and Elixir wrapper have been patched to be compatible with the new XGBoost C API, modern compilers, and updated data formats.
• Dependency & Deprecation Cleanup: All dependencies have been updated, and deprecation warnings related to charlists and VegaLite have been resolved.

The library now compiles cleanly and passes its entire test suite on a modern Erlang (28) and Elixir (1.18) development environment.

Precompiled Binaries (GH workflows)

• Updated runner to use ubuntu-24.04
• Dropped Erlang 25.0, 26.0; Elixir 1.14 for Erlang 27/28 and Elixir 1.18
• Dropped Intel Mac support (they did build successfully -- happy to add back)

[acalejos]

@cdmwebs Thanks so much for this PR! I'll try to take a look this weekend

[javiercr]

This is fantastic! Thank you so much for your effort in keeping this library up to date 🙏

[cdmwebs]

We use it everyday. Happy to help.

[iperks]

@acalejos do you think this is good to merge?