Update C++ standard requirement from C++17 to C++20

The project uses C++20 features (concepts, likely/unlikely attributes, etc.)
as noted in CHANGES.md. Updated prerequisite documentation to reflect this:

- CONTRIBUTING.md: Updated compiler requirements to C++20 (GCC 10+, Clang 10+, MSVC 2019+)
- docs/DEVELOPMENT.md: Updated compiler requirements to C++20

This ensures developers are aware of the correct C++ standard required
for building the project.

Author: claude

CONTRIBUTING.md

@@ -8,7 +8,7 @@ Thank you for your interest in contributing to python_prtree!
 
 - Python 3.8 or higher
 - CMake 3.12 or higher
-- C++17-compatible compiler (GCC 7+, Clang 5+, MSVC 2017+)
+- C++20-compatible compiler (GCC 10+, Clang 10+, MSVC 2019+)
 - Git
 
 ### Quick Start

docs/DEVELOPMENT.md

@@ -36,7 +36,7 @@ For a detailed explanation of the architecture, see [ARCHITECTURE.md](ARCHITECTU
 
 - Python 3.8 or higher
 - CMake 3.22 or higher
-- C++17 compatible compiler
+- C++20 compatible compiler (GCC 10+, Clang 10+, MSVC 2019+)
 - Git (for submodules)
 
 ### Platform-Specific Requirements