(eieio-backward-compatibility): Set to `warn` (bug#77612)

(eieio-backward-compatibility): Set to `warn` (bug#77612)

* lisp/emacs-lisp/eieio-base.el (make-instance) <eieio-named>:
Really skip backward compatibility when `eieio-backward-compatibility`
is nil and emit message if it's `warn`.
(eieio-persistent-make-instance): Warn when an obsolete name is used.

* lisp/emacs-lisp/eieio-core.el (eieio-backward-compatibility):
Change default to `warn`.
(eieio-defclass-internal): Warn when the *-list-p function is called
(eieio--slot-name-index): Warn when a initarg is used to access a slot.

* lisp/emacs-lisp/eieio.el (defclass): Warn when a class-slot is
accessed via the obsolete method.
(make-instance, clone) <eieio-default-superclass>: Really skip backward
compatibility when `eieio-backward-compatibility` is nil and emit
message if it's `warn`.

(cherry picked from commit ae1d01328f260f1a9891280c96139494629a83e8)