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/backend/access/common/reloptions.c

@@ -461,6 +461,15 @@ static relopt_real realRelOpts[] =
 		},
 		0, -1.0, DBL_MAX
 	},
+	{
+		{
+			"vacuum_cleanup_index_scale_factor",
+			"Deprecated B-Tree parameter.",
+			RELOPT_KIND_BTREE,
+			ShareUpdateExclusiveLock
+		},
+		-1, 0.0, 1e10
+	},
 	/* list terminator */
 	{{NULL}}
 };