bug#55862: Removing indent-tabs-mode set to t by default

[Drew Adams <drew.adams@oracle.com>]

> It being a long-time default of ours has no effect on whether using
> tabs is a good idea (since Emacs is not particularly popular these days to
> affect global practive).
> 
> But you know my opinion on this anyway.

As long as reasons are being given, let me add this one:

People copy bits of code and paste that text into places
other than Emacs - online Q&A forums, email, etc.

TAB chars here and there due to `indent-tabs-mode' force
users to manually reformat the code - or at least to have
known about and thought to use `M-x untabify' beforehand.

This ex-Emacs use case is likely larger today than it was
decades ago.  Defaulting the var to t just produces more
bother and annoyance, especially for newbies (who are
less likely to use `untabify').

Just one more reason why the current default is not ideal.

Not that posting this here will help.  Except that it
might bring it to the attention of a few readers to whom
it might not have occurred.