Fix for #49 and verification against liboqs 0.15.0

[jplomas]

Rationale

The liboqs-upstream-trigger job has been failing for many months. This PR verifies liboqs-go with the latest liboqs release and fixes #49

Notes for Reviewers

This pull request updates the project to support liboqs version 0.15.0 and makes several important changes to documentation and build workflows. The most significant changes include updating version numbers, reflecting upstream cryptographic algorithm removals and additions, and improving the GitHub Actions workflow for both POSIX and Windows environments.

Version and Compatibility Updates:

• Updated all relevant version numbers and documentation to reference liboqs 0.15.0, including .config/liboqs-go.pc, .config/liboqs-go.pc.linux, .config/liboqs-go.pc.win64, RELEASE.md, and CHANGES.md.

Build and Continuous Integration Improvements:

• Refactored the GitHub Actions workflow to separate POSIX and Windows builds, improving clarity and maintainability. The Windows build now uses MSYS2/MinGW for better compatibility (as per the README, as liboqs was defaulting to building with MSVC), and the test timeout has been increased to 30 minutes (required for SLH-DSA tests).
• Updated linker flags for Windows in .config/liboqs-go.pc.win64 to use the correct stack size syntax for the platform.

Documentation Enhancements:

• Expanded and clarified the changelog in CHANGES.md, including details about cryptographic algorithm support and migration notes for users.
• Updated the functional restrictions section in README.md to match the new liboqs version and algorithm support status.