Address Emacs 27 warnings

* lisp/org-entities.el (seq): Require seq.el - it is not preloaded in
Emacs <28.
* lisp/org.el (org--image-yank-media-handler): Suppress warning and
throw an error about `file-name-with-extension'.  This function is not
supposed to be called in older Emacs.

lisp/org-entities.el

@@ -29,6 +29,7 @@
 
 (require 'org-macs)
 (org-assert-version)
+(require 'seq) ; Emacs 27 does not preload seq.el; for `seq-every-p'.
 
 (declare-function org-mode "org" ())
 (declare-function org-toggle-pretty-entities "org"       ())

lisp/org.el

@@ -20580,11 +20580,13 @@ It is saved as per `org-yank-image-save-method'.  The name for the
 image is prompted and the extension is automatically added to the
 end."
   (cl-assert (fboundp 'mailcap-mime-type-to-extension)) ; Emacs >=29
+  (cl-assert (fboundp 'file-name-with-extension)) ; Emacs >=28
   (let* ((ext (symbol-name
                (with-no-warnings ; Suppress warning in Emacs <29
                  (mailcap-mime-type-to-extension mimetype))))
          (iname (funcall org-yank-image-file-name-function))
-         (filename (file-name-with-extension iname ext))
+         (filename (with-no-warnings ; Suppress warning in Emacs <28
+                     (file-name-with-extension iname ext)))
          (absname (expand-file-name
                    filename
                    (if (eq org-yank-image-save-method 'attach)