Deprecate webxdc sendUpdate's desc parameter

[adbenitez]

as commented in #6230 (comment)

the desc field is unnecessary, was never needed/used by other implementators that raised questions about it, ex. in XMPP, it was a DeltaChat-specific thing mainly, to show something nice to a classic email user, but that doesn't make much sense anyways as status updates can be a lot and are internal data not supposed to be analyzed or described manually

• so we should keep it but deprecate it, don't use it internally
• the webxdc specs need to be updated
• the UIs need to update their webxdc.js file to not fail if the desc argument is not given

this is a backwards compatible change since in javascript you can call functions with arbitrary amount of extra arguments and they will just be ignored/unused