Remove python2 guidance

[johanste]

Two sets of guideline changes: (1) remove all Python 2 compatibility guidance and update typing for a 3.9+ minimum, and (2) restrict **kwargs usage to pass-through scenarios only.

Python 2 removal & typing modernization (design.md, implementation.md)

• Minimum version: 3.8 → 3.9
• Removed azure-nspkg / azure-<group>-nspkg dependency requirements
• Removed collections fallback (Python 2.7), "straddling code" PEP 484 reference, (object) base class from examples, "only need to support Python 3" qualifiers, Python 3.4/3.5 historical references
• Added PEP 585 requirement — use list[str] not typing.List[str]
• Added Union/Optional guidance with comment noting PEP 604 X | Y syntax for when minimum moves to 3.10+

# 3.9+
def get_things() -> list[str]: ...
def foo(x: Union[int, str]) -> Optional[str]: ...

# When minimum is 3.10+:
# def foo(x: int | str) -> str | None: ...

Keyword-only arguments over **kwargs (implementation.md, design.md, documentation.md)

• Added MUSTNOT (python-codestyle-no-kwargs-for-arguments): **kwargs must not be used for arguments consumed by the method — use keyword-only args
• Added MAY (python-codestyle-kwargs-passthrough): **kwargs allowed only for pass-through, must document target API(s)
• Updated docstring requirements in both design.md and documentation.md to require documenting which API(s) receive forwarded **kwargs
• Removed **kwargs as alternative in python-codestyle-long-args

# Yes — explicit keyword-only arguments:
def create_thing(name: str, *, size: int = 0, color: str = "blue") -> None: ...

# No — kwargs.pop pattern for consumed arguments:
def create_thing(name: str, **kwargs) -> None:
    size = kwargs.pop("size", 0)
    ...

# OK — pass-through with documented target:
def get_thing(self, name: str, **kwargs) -> "Thing":
    """Keyword arguments are passed to the HTTP pipeline.
    See https://aka.ms/azsdk/python/options.
    """
    return self._pipeline.send(request, **kwargs)