Add .NET 8 support for Authentication.JwtBearer.Google #201

In .NET 8 there is a breaking change that the Authentication.JwtBearer.Google is affected by.

At the very least, you should include documentation specifying that UseSecurityTokenValidators=true is now a required configuration.

.AddJwtBearer(options => 
{
    options.UseSecurityTokenValidators = true;
    options.UseGoogle("...");
});