Deprecate Adyen integration

As we did in the past for Ogone and Authorize.net, deprecate Adyen payment gateway and remove the option for choosing it in (Admin portal -> Settings -> Billing). It will continue active for accounts already using it but they won't be able to switch back if they change it (show message).

Update documentation and remove the Adyen chapter inside the billing section.

We will announce this deprecation in 2.6 release notes. The plan is to completely remove this and drop support in 2.7.

Dev notes

This is easy. Just add deprecated: true on this line https://github.com/3scale/porta/blob/2914c8135c2ddd1dd3ef484ac2ada69cda164922/app/models/payment_gateway.rb#L18

PR where the way to deprecate payment gateways was first introduced: https://github.com/3scale/system/pull/9295.

Though some tests might fail and need to be fixed but no difficulty at all