core: bump version to 4.0.0-dev, follow "practical" semantic versioning

src/gui/gui-key.c

@@ -301,7 +301,7 @@ gui_key_grab_end_timer_cb (const void *pointer, void *data, int remaining_calls)
 /*
  * Gets internal code from user key name.
  *
- * Note: this function works with legacy keys (WeeChat < 3.9) and should not
+ * Note: this function works with legacy keys (WeeChat < 4.0.0) and should not
  * be used any more.
  *
  * Examples:

src/plugins/irc/irc-upgrade.c

@@ -418,7 +418,7 @@ irc_upgrade_read_cb (const void *pointer, void *data,
                     }
                     /*
                      * "authentication_method" and "sasl_mechanism_used" are
-                     * new in WeeChat 3.9
+                     * new in WeeChat 4.0.0
                      */
                     if (weechat_infolist_search_var (infolist, "authentication_method"))
                     {