Better push text for group chats (de)

What does this MR do?

The german translation for group push notifications currently says:

{foodsaver} hat dir in {conversation} geschrieben.

I think the "dir" is out of place here, because in the default case, messages in group chats are not specifically addressed to every receiver of the notification individually.

Even though the translation key only refers to "named" conversations, these seem to be exclusively conversations with > 2 members[1], therefore group chats.

[1] https://gitlab.com/foodsharing-dev/foodsharing/-/blob/master/src/Services/MessageService.php#L133

How confident are you it won't break things if deployed?

very

Links to related issues

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

(if they really want. this only touches translations)

1. Checkout branch locally
2. Login as any user
3. enable push notifications (do they work in dev?)
4. Login as different user in a group chat with the previous user (using a different browser/session)
5. Send a message
6. Verify the notification text has changed

Screenshots (if applicable)

Any relevant screenshots if this is a design / frontend change

Checklist

• no test necessary, just changed a translation string
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings