tidb_opt_limit_push_down_threshold need to be named to match its actual function.

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

From https://github.com/pingcap/tidb/issues/63370,
tidb_opt_limit_push_down_threshold should only impacts TopN pushdown, not limit pushdown.
Limit pushdown should always happen.
This is inconsistent with the description in doc:https://docs.pingcap.com/tidb/stable/system-variables/#tidb_opt_limit_push_down_threshold

### 2. What did you expect to see? (Required)
System variable name should be consistent with actual functions

### 3. What did you see instead (Required)
not consistent now
### 4. What is your TiDB version? (Required)
master