Update Adyen Web Drop-in from version `3.23.0` to `5.14.0`

[remcotolsma]

Documentation

• https://docs.adyen.com/online-payments/web-drop-in/integrated-before-5-0-0
• https://docs.adyen.com/online-payments/web-drop-in

To-do

• [x] Remove old web SDK gateway. https://github.com/pronamic/wp-pronamic-pay-adyen/blob/3.1.0/src/WebSdkGateway.php
• [x] Remove "API-key" setting, use new clientKey parameter. https://github.com/pronamic/wp-pronamic-pay-adyen/blob/3.1.0/src/Integration.php#L226-L240
• [x] Remove "Origin Key" setting, no longer used, use new clientKey parameter. https://github.com/pronamic/wp-pronamic-pay-adyen/blob/3.1.0/src/Integration.php#L260-L278
• [x] Add new clientKey setting.
• [x] Add new environment setting: test, live, live-au, etc. https://docs.adyen.com/online-payments/web-drop-in#test-and-go-live
• [x] https://github.com/pronamic/wp-pronamic-pay-adyen/issues/10 As far as I'm concerned we spend very little time on this, maybe a notification and a major version update (4.0).

Related issues

• [x] https://github.com/pronamic/wp-pronamic-pay-adyen/issues/1
• [x] https://github.com/pronamic/wp-pronamic-pay-adyen/issues/3

[remcotolsma]

• [x] Remove old web SDK gateway. https://github.com/pronamic/wp-pronamic-pay-adyen/blob/3.1.0/src/WebSdkGateway.php

Removed in https://github.com/pronamic/wp-pronamic-pay-adyen/commit/348782efcbfc3a4dbf7e7b9167161702e86129ea.

• [x] Remove "API-key" setting, use new clientKey parameter. https://github.com/pronamic/wp-pronamic-pay-adyen/blob/3.1.0/src/Integration.php#L226-L240

API-key is probably still needed for server side communication with Adyen.

• [x] Remove "Origin Key" setting, no longer used, use new clientKey parameter. https://github.com/pronamic/wp-pronamic-pay-adyen/blob/3.1.0/src/Integration.php#L260-L278
• [x] Add new clientKey setting.

Removed/added in https://github.com/pronamic/wp-pronamic-pay-adyen/commit/c3d9bf2764b477dbbfe507bcad97106583711965.