Backport from main: Fix compatibility with Emacs 26

* lisp/org-compat.el (org-replace-buffer-contents): Add compatibility
function for `replace-buffer-contents'.

* lisp/org-src.el (org-edit-src-save, org-edit-src-exit): Use
`org-replace-buffer-contents'.

lisp/org-compat.el

@@ -113,6 +113,11 @@ the symbol of the calling function, for example."
 
 ;;; Emacs < 27.1 compatibility
 
+(if (version< emacs-version "27.1")
+    (defsubst org-replace-buffer-contents (source &optional _max-secs _max-costs)
+      (replace-buffer-contents source))
+  (defalias 'org-replace-buffer-contents #'replace-buffer-contents))
+
 (unless (fboundp 'proper-list-p)
   ;; `proper-list-p' was added in Emacs 27.1.  The function below is
   ;; taken from Emacs subr.el 200195e824b^.

lisp/org-src.el

@@ -1235,7 +1235,7 @@ Throw an error if there is no such buffer."
 		   (insert (with-current-buffer write-back-buf (buffer-string))))
 	  (save-restriction
 	    (narrow-to-region beg end)
-	    (replace-buffer-contents write-back-buf 0.1 nil)
+	    (org-replace-buffer-contents write-back-buf 0.1 nil)
 	    (goto-char (point-max))))
 	(when (and expecting-bol (not (bolp))) (insert "\n")))
       (kill-buffer write-back-buf)
@@ -1283,7 +1283,7 @@ Throw an error if there is no such buffer."
                               (buffer-string))))
 	   (save-restriction
 	     (narrow-to-region beg end)
-	     (replace-buffer-contents write-back-buf 0.1 nil)
+	     (org-replace-buffer-contents write-back-buf 0.1 nil)
 	     (goto-char (point-max))))
 	 (when (and expecting-bol (not (bolp))) (insert "\n")))))
     (when write-back-buf (kill-buffer write-back-buf))