Reduce sync query size for expedient amelioration of #22636 (#22662)

It's too slow, it's O(n), make n go down a fair bit while we do a bigger fix and set up regular profiling just for this

src/metabase/driver/sql_jdbc/sync/describe_table.clj

@@ -206,7 +206,7 @@
 
 (def ^:const nested-field-sample-limit
   "Number of rows to sample for describe-nested-field-columns"
-  10000)
+  500)
 
 (defn- flattened-row [field-name row]
   (letfn [(flatten-row [row path]