[php] Only use values up to the second accuracy for PHP datetime values

Created by: carmenquan

https://github.com/OpenAPITools/openapi-generator/issues/10548

Description PHP is unable to handle highly precise DateTime Values. While this issue covered most cases, we have a few instances of DateTime which are not handled. (i.e. "2021-10-06T20:17:16.076372256Z")

Example:

print_r(strtotime("2021-10-06T20:17:16.076372256Z")); // => ""
print_r(strtotime("2021-10-06T20:17:16.07637225Z")); // =>  "1633551436"

To fix this we only use up to the second accuracy by trimming the date string. strtotime is only accurate up to number of seconds so this does not result in data loss. https://www.php.net/manual/en/function.strtotime.php

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