[CDRIVER-6017] Reduce BSON_VALIDATION_MAX_NESTING_DEPTH to 500 (#2036)

To fix stack overflow encountered on MSVC on r1.30 branch. Likely caused by default over-alignment of `bson_t` and `bson_iter_t` removed in 2.0.

Author: kevinAlbs

src/libbson/src/bson/validate-private.h

@@ -15,7 +15,7 @@ enum {
     * server might accept. The main purpose of this limit is to prevent stack
     * overflow, not to reject invalid data.
     */
-   BSON_VALIDATION_MAX_NESTING_DEPTH = 1000,
+   BSON_VALIDATION_MAX_NESTING_DEPTH = 500,
 };
 
 /**