gsubprocess: Fix ‘Since’ lines in documentation to read 2.40

GSubprocess was introduced in GLib 2.40, not 2.36.

https://bugzilla.gnome.org/show_bug.cgi?id=724001

gio/gioenums.h

@@ -1744,7 +1744,7 @@ typedef enum /*< flags >*/ {
  * example, you may not request both %G_SUBPROCESS_FLAGS_STDOUT_PIPE and
  * %G_SUBPROCESS_FLAGS_STDOUT_SILENCE.
  *
- * Since: 2.36
+ * Since: 2.40
  **/
 typedef enum {
   G_SUBPROCESS_FLAGS_NONE                  = 0,

gio/giotypes.h

@@ -518,7 +518,7 @@ typedef struct _GTestDBus GTestDBus;
  *
  * A child process.
  *
- * Since: 2.36
+ * Since: 2.40
  */
 typedef struct _GSubprocess                   GSubprocess;
 /**