Add back vacuum_cleanup_index_scale_factor parameter.

Commit 9f3665fb removed the vacuum_cleanup_index_scale_factor storage
parameter.  However, that creates dump/reload hazards when moving across
major versions.

Add back the vacuum_cleanup_index_scale_factor parameter (though not the
GUC of the same name) purely to avoid problems when using tools like
pg_upgrade.  The parameter remains disabled and undocumented.

No backpatch to Postgres 13, since vacuum_cleanup_index_scale_factor was
only disabled by REL_13_STABLE's version of master branch commit
9f3665fb in the first place -- the parameter already looks like this on
REL_13_STABLE.

Discussion: https://postgr.es/m/YEm/a3Ko3nKnBuVq@paquier.xyz

src/include/access/nbtree.h

@@ -1067,6 +1067,7 @@ typedef struct BTOptions
 {
 	int32		varlena_header_;	/* varlena header (do not touch directly!) */
 	int			fillfactor;		/* page fill factor in percent (0..100) */
+	float8		vacuum_cleanup_index_scale_factor; /* deprecated */
 	bool		deduplicate_items;	/* Try to deduplicate items? */
 } BTOptions;