Constant folding of to_timestamp uses different implementation than actual function call

[alamb]

Describe the bug
A clear and concise description of what the bug is.
Different implementations of string --> timestamp conversions are used if to_timestamp() is called in constant folding pass or in the actual compute kernel

To Reproduce
The function string_to_timestamp_nanos was moved upstream to arrow-rs and is used in constant folding here: https://github.com/apache/arrow-datafusion/blob/ffb195c8634fc537127eff1a082811d28e8fcc2b/datafusion/src/optimizer/constant_folding.rs

use arrow::compute::kernels::cast_utils::string_to_timestamp_nanos;

However, there is still an older version of string_to_timestamp_nanos that is in the datafusion codebase:
https://github.com/apache/arrow-datafusion/blob/5900b4c6829b0bdbe69e1f95fb74e935bc8f33d4/datafusion/src/physical_plan/datetime_expressions.rs#L269

Expected behavior
datafusion should use the same function to convert between string -> timestamps in both places

Additional context
It appears the functions were moved in apache/arrow-rs@db030cd but did not quite clean up the datafusion implementation