Rename "private messages" to "direct messages"

[alya]

In zulip/zulip#24224 and follow-up work, we renamed "private messages" to "direct messages" in the web app. We should do the same in the mobile app. This issue is specifically for user-facing stings.

The mobile instructions on /help/direct-messages and in /help/include/send-group-dm.md will need to be updated accordingly, either as part of this issue, or as an immediate follow-up.

[gnprice]

This issue is specifically for user-facing strings.

Just to underline this part: let's defer any renames or other refactoring, leaving non-user-facing uses of "private" alone for now.