Stop referring to GDExtension as experimental in the README #428

Job	Run time
📊 Static Checks / Format (clang-format, ruff format, file format)	14s
🛠️ Continuous Integration / 🏁 Build (Windows, MSVC, CMake)	7m 43s
🛠️ Continuous Integration / 🐧 Build (Linux, GCC, CMake Ninja)	5m 44s
🛠️ Continuous Integration / 🐧 Linux (GCC)	1m 8s
🛠️ Continuous Integration / 🐧 Linux (GCC, Double Precision)	49s
🛠️ Continuous Integration / 🏁 Windows (x86_64, MSVC)	1m 50s
🛠️ Continuous Integration / 🏁 Windows (x86_64, MinGW)	39m 7s
🛠️ Continuous Integration / 🍎 macOS (universal)	45s
🛠️ Continuous Integration / 🤖 Android (arm64)	1m 9s
🛠️ Continuous Integration / 🍏 iOS (arm64)	41s
🛠️ Continuous Integration / 🌐 Web (wasm32)	1m 10s
	1h 0m 20s

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