#7116 may potentially break group-addition if one side uses an older Delta Chat version.

Steps to reproduce:

• Bob uses a Delta Chat version 2.20 or older (i.e. Delta Chat without the Calls release, which is making it to the stores these days)
• Alice uses core with Remove protected chats and expire auth tokens #7116
• Alice adds a new member to a group
• Since Remove protected chats and expire auth tokens #7116 we don't send the Chat-Verified header anymore, so, Bob doesn't accept the new member as verified
• -> When the new member sends a message into the group Bob's device replaces it with an error message [The message was sent by non-verified contact. Re-download the message or see 'Info' for more details].
• -> The solution for Bob is to update Delta Chat, then he'll see the messages normally again.

We want to improve backwards compatibility by sending a Chat-Verified header iff:

• the group chat was protected before (can be checked with SELECT protected FROM chats WHERE id=?)
• AND all members are verified

Then member-additions can still fail if Alice doesn't have the new member as verified, but this will be a lot rarer.