docs: release 1.2.0 and unpin downstream from the 1.1 series

Issue #10 Task 6. The install pin moves from ==1.1.* to >=1.2,<2 - left
alone, everyone following the README would have stayed silently on
1.1.2 without this fix and without a warning. Telemetry field count
re-measured via inspect.signature: still 22.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "polygateway"
-version = "1.1.2"
+version = "1.2.0"
 description = "PolyGateway：实验室统一的大语言模型（LLM/VLM/OCR）调度与中转库——多源、限流、重试、熔断、缓存、遥测"
 # registry 包页面的正文只认这一项:缺了页面就是一片空白(1.1.2 的教训,twine 会警告
 # long_description missing 但不阻塞上传)。README 在打包时被固化进产物,发布后再改无效。