Deprecate SOAP support

[Gold]

While working through PHPStan issues I found that the SOAP implementation should have been reliably throwing errors. The lack of issues in this area suggests that this is not being used at all.

Rather than supporting what is now quite old code we should just deprecate it and remove it from the codebase.

The area that drew this to my attention is here:

https://git.mahara.org/mahara/mahara/-/blob/main/htdocs/lib/mahara.php#L2640

$wsseSoapClient = new webservice_soap_client_wsse(
  array($client, '_doRequest'),
  $client->wsdlfile,
  $client->getOptions()
);

The first parameter, which should declare our request, is unused in webservice_soap_client_wsse()
The third parameter is unused also. In the webservice_soap_client_wsse() class there is a private $options, but this is never set.

Also, webservice_soap_client(), which gives us $client, has no getOptions() method so this should also be erroring out loudly.

[Kristina Hoeppner (kris-hoeppner)]

In the first instance: Add deprecation message that it will go away in 24.04.

[Mahara Bot (dev-mahara)]

Patch for "main" branch: https://reviews.mahara.org/c/mahara/+/13885