DEPR: Positional arguments in Series.to_string
- [X] closes #57280
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [X] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [X] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

@mroeschke Pinging on green Looking good! You can already make the two overloads keyword-only, if you want to:

```py
    @overload
    def to_string(
        self,
        buf: None = ...,
        *,
        na_rep: str = ...,
``` @twoertwein It looks like the two overloaded methods are already keyword only. Is there something specific I need to do? > Is there something specific I need to do?

Just need to add two `*,`

```py
    @overload
    def to_string(
        self,
        buf: None = ...,
       *,  # add this
        na_rep: str = ...,

    @overload
    def to_string(
        self,
        buf: FilePath | WriteBuffer[str],
       *, # add this
        na_rep: str = ...,

    @deprecate_nonkeyword_arguments(
    def to_string(
``` @twoertwein Pinging on green. 

BTW I didn't know that you can use `*` to mandate the use of keyword args after the `*`. I learned something new today! Thank you @rmhowe425 !
