[SPARK-16960][SQL] Deprecate approxCountDistinct, toDegrees and toRadians...

[SPARK-16960][SQL] Deprecate approxCountDistinct, toDegrees and toRadians according to FunctionRegistry

## What changes were proposed in this pull request?

It seems `approxCountDistinct`, `toDegrees` and `toRadians` are also missed while matching the names to the ones in `FunctionRegistry`. (please see [approx_count_distinct](https://github.com/apache/spark/blob/5c2ae79bfcf448d8dc9217efafa1409997c739de/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala#L244), [degrees](https://github.com/apache/spark/blob/5c2ae79bfcf448d8dc9217efafa1409997c739de/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala#L203) and [radians](https://github.com/apache/spark/blob/5c2ae79bfcf448d8dc9217efafa1409997c739de/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala#L222) in `FunctionRegistry`).

I took a scan between `functions.scala` and `FunctionRegistry` and it seems these are all left. For `countDistinct` and `sumDistinct`, they are not registered in `FunctionRegistry`.

This PR deprecates `approxCountDistinct`, `toDegrees` and `toRadians` and introduces `approx_count_distinct`, `degrees` and `radians`.

## How was this patch tested?

Existing tests should cover this.

Author: hyukjinkwon <gurwls223@gmail.com>
Author: Hyukjin Kwon <gurwls223@gmail.com>

Closes #14538 from HyukjinKwon/SPARK-16588-followup.