typing docs: Use soft deprecation

A while ago we introduced the concept of "soft deprecation" in PEP 387 (https://peps.python.org/pep-0387/#soft-deprecation): for things that we no longer recommend using, but which we aren't planning to remove in the near future.

The typing docs already list four classes of objects that are deprecated without a scheduled removal (https://docs.python.org/3.14/library/typing.html#deprecation-timeline-of-major-features):

• Pre-PEP-585 generic aliases (e.g. List)
• Text
• Hashable and Sized
• TypeAlias

I'd like to also soft-deprecate the following:

• Optional (obviated by PEP-604)
• NoReturn (prefer Never)
• typing.ForwardRef (deprecated alias for annotationlib.ForwardRef)
• types.UnionType (deprecated alias for typing.Union)
• Creating a Union using subscripting (Union[A, B] instead of A | B) (PEP-604)
• Creating a TypeVar, ParamSpec, or TypeVarTuple using the constructor directly (PEP-695)
• Directly inheriting from Generic (PEP-695)

None of those will be removable for many years if ever, but I think it's useful to have a clear statement in the docs that the newer syntax is preferred.

Linked PRs

• gh-154480

从所链接的 Python 文档页面中标题为“Deprecation timeline of major features”的 typing 文档部分开始，并将其中现有的条目与所请求的 soft deprecations 进行比较。当文档清楚地标识出列出的旧形式并推荐其更新的替代形式时，即视为完成；请先查看链接的 PR gh-154480，因为相关工作已经与它关联。