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

リンク先の Python ドキュメントページにある、タイトルが “Deprecation timeline of major features” の typing ドキュメントセクションから始め、既存の項目を、要求されている soft deprecations と比較してください。ドキュメントが一覧にある古い形式を明確に特定し、それらの新しい代替手段を推奨していれば完了です; 作業はすでに関連付けられているため、まずリンク先の PR gh-154480 を確認してください。