In 2021 in 2f02bc90 (demo: URI resolver) we added support for generic
demo:// URI scheme which allows combining any two storages into a
DemoStorage. This support is more generic because e.g. previously
?demostorage was provided only by file:// and zeo:// URI schemes, with
the "changes" part there being hardcoded to in-memory MappingStorage.

Now there is no need to use ?demostorage option as we can use demo://
uniformly.

Deprecating ?demostorage in favour of dedicated DemoStorage resolver was
actually suggested several times in the past:

https://github.com/Pylons/zodburi/pull/25#issuecomment-485506959
https://github.com/Pylons/zodburi/pull/25#issuecomment-511480572
https://github.com/Pylons/zodburi/pull/25#issuecomment-622931793

/reviewed-by @stevepiercy, @tseaver
/reviewed-on https://github.com/Pylons/zodburi/pull/33