fix constructor used in ECDSA-SHA256 algorithm

[hirasawayuki]

Fix ECDSA-SHA256 algorithm to use create() constructor.

When I call SWA\TokenBuilder#getClientSecret(), I get the following error message:

TypeError: Too few arguments to function Lcobucci\JWT\Signer\Ecdsa::__construct(), 0 passed in /auth/vendor/ktakayama/swa/src/SWA/TokenBuilder.php on line 64 and exactly 1 expected

This error seems to be caused by not using the create() constructor with the ECDSA-SHA256 algorithm.
According to the official documentation:

The implementation of ECDSA algorithms have a constructor dependency. Use the create() named constructor to avoid having to handle it (e.g.: Lcobucci\JWT\Signer\Ecdsa\Sha256::create()).

https://lcobucci-jwt.readthedocs.io/en/latest/configuration/#:~:text=arguments%20here%0A)%3B-,Important,-The%20implementation%20of