bug#75306: 31.0.50; Make `small-temporary-file-directory` variable obsolete

["Björn Bidar via Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Michael Albinus via "Bug reports for GNU Emacs, the Swiss army knife of
text editors" <bug-gnu-emacs@gnu.org> writes:

> Stefan Kangas <stefankangas@gmail.com> writes:
>
> Hi Stefan,
>
>> I think the variable doesn't make much sense these days, and should be
>> made obsolete.  Note that it is barely used in Emacs and third-party
>> packages.
>>
>> However, Tramp recently started using the variable as a place to create
>> OpenSSH Unix domain sockets.  May I suggest that Tramp uses some other
>> variable for this purpose, perhaps a Tramp specific one?
>
> Is there a reason that it must be changed? What are the advantages? Does
> it hurt to keep the user option?
>
> And in the Tramp case, you break an existing interface. Users have been
> instructed to change the value of small-temporary-file-directory when
> needed. See (info "(tramp) Frequently Asked Questions"), which documents
> a user problem reported on the <tramp-devel@gnu.org> ML.

If the intend of the variable is the same as listed in the Tramp FAQ why
not set it by default to /run/user/<uid>?

For such use cases using /run/user/<uid> is much better as it more
secure and on a ramdisk.