Update the default hashing algorithm used in signed cookies

[javiereguiluz]

This bundle uses sha256 as the default hashing algorithm for signed cookies:

NelmioSecurityBundle/src/DependencyInjection/Configuration.php

Line 255 in 93f8009

->scalarNode('hash_algo')->defaultValue('sha256')->end()

This might be no longer safe enough: https://en.wikipedia.org/wiki/SHA-2#Comparison_of_SHA_functions

Maybe we can move to sha3-256 or higher, which is also supported by PHP? https://www.php.net/manual/en/function.hash-algos.php