vllm-flash-attn: allow Python 3.14 in the supported-versions gate.

The extension uses Python's stable ABI, so the hard-coded configure-time
whitelist can safely follow vllm's own Python compatibility declaration.

Tested against vllm-project/flash-attention @ 803020a (vllm-0.24.0 pin).

--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -22 +22 @@
-set(PYTHON_SUPPORTED_VERSIONS "3.9" "3.10" "3.11" "3.12" "3.13")
+set(PYTHON_SUPPORTED_VERSIONS "3.9" "3.10" "3.11" "3.12" "3.13" "3.14")