Make newer ElDoc versions are compatible with Emacs < 28

Make newer ElDoc versions are compatible with Emacs < 28

Fixes: bug#42563
For some time, Eldoc has has some Elisp-specific code that shouldn't
live there, but in elisp-mode.el.  This can be fixed in Emacs master,
but since ElDoc is distributed in GNU Elpa and is meant to work with
Emacs 26 and 27, this means that that elisp-specific code must still
be distributed with eldoc.el and kept up to date.

* lisp/emacs-lisp/eldoc.el (eldoc--eval-expression-setup): Ensure
implementation is compatible with Emacs < 28.
(Version): Bump to 1.8