Rename the string kernel to concatenate_elements

[HaoYang670]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
This is a follow-up of #1720.
We have added concatenate by elements support for string array and put it into the new created string kernel.
I suggest we should rename the kernel concatenate_elements.

There are some reasons I thought why concatenate_elements is better than string:

1. The name of the compute kernel should express what kind of operation will be done on an array, such as cast, aggregate ...
2. We can support the concatenate_elements operation on binary array, list array and other nested types. So string is not an appropriate name.
3. In cudf (which is based on Arrow c++, and more GPU supported), there are similar kernels and we can refer to their names.: https://github.com/rapidsai/cudf/blob/branch-22.08/cpp/src/lists/combine/concatenate_list_elements.cu
   https://github.com/rapidsai/cudf/blob/branch-22.08/cpp/src/lists/combine/concatenate_rows.cu

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[HaoYang670]

cc @alamb @tustvold @viirya. Looking forward to your opinions!

[alamb]

Hi @HaoYang670

I suggest we should rename the kernel concatenate_elements.

If you mean rename arrow/src/compute/kernels/string.rs to arrow/src/compute/kernels/concat_elements.rs I think it is a great idea (and ideally we could do it in the next day or two before we released arrow 15 avoiding a breaking change in a released version)