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ARROW-13684: [C++][Compute] Strftime kernel follow-up

This is to resolve [ARROW-13684](https://issues.apache.org/jira/browse/ARROW-13684).

1. Default strftime string is now `%Y-%m-%dT%H:%M:%S`. Perhaps `%Y-%m-%dT%H:%M:%S%z` would be better?
2. Timestamps without timezone are now strftime-ed as if they were in `UTC`. Not sure this is the way to go. What if the local time is really invalid but we can't tell?
3. Document `%S` behavior. What would be a good location to do that?

Closes #10998 from rok/ARROW-13684

Lead-authored-by: Rok <rok@mihevc.org>
Co-authored-by: Rok Mihevc <rok@mihevc.org>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>