Based on #7147

Since key-contacts cannot change their keys and having "verification" is less important for contact identification than having a chat history or shared chats with a contact, UIs have stopped displaying green checkmarks everywhere (deltachat/deltachat-android#3828).

There was also a bug #7107 (closed in #7113) that resulted in updating "verified by" information for already verified contacts, so the information about "verifiers" is lost.

We also want to make "verification" more meaningful by only verifying contacts if contact QR code is scanned directly within a short period of time and not just because someone used an invite link to join the group: #7111

Because of this it makes sense to reset existing verification status for contacts and start from scratch.

We can also allow to remove verification from contacts (https://support.delta.chat/t/how-would-i-remove-the-verified-checkmarks-from-one-invite-code-in-retrospect/3403) as it will not break "protected" chats anymore, but this is out of scope for this PR.

The plan is to stop creating new "protected" chats, convert already "protected" group chats into just group chats, reset existing verifications and switch to a new mechanism for verification gossip so old verifications don't resurrect via gossip.

What this PR does:

• Remove is_protection_broken APIs. (moved to Various protected group changes #7146)
• Remove is_profile_verified API. (moved to Various protected group changes #7146)
• Stop indirectly verifying contacts via messages with a Chat-Verified: 1 header.
• Add new _verified attribute for Autocrypt-Gossip and use it to indirectly verify key-contacts when such attribute is received from a verified contact (whether directly verified or not).
• Make sure that direct verifications cannot happen via "gossip" from self via outgoing messages sent by other devices. This is another reason to reset existing verifications, apparently existing "direct" verifications may be not actually direct but gossiped by own devices.
• Remove error paths from add_parts that result in "The message was sent by non-verified contact" messages. We likely don't need to pass verified_encryption around, it is only used to decide whether we want to accept verification gossip from the contact.
• Remove Chat.is_protected() APIs.
• Remove API to create new protected groups
• Update dc_contact_is_verified documentation.

To close #7111 we also need to:

• Expiring QR codes? Not sure we want to do this in the same PR. It will make verifications more meaningful though, so ideally should be done now too.
• Reset existing verifications via migration. We should do this at the same time as we switch to the new gossiping mechanism, so it should be part of this PR.

Closes #7080 (replace verification gossip mechanism with a new one that works using Autocrypt-Gossip header and is independent of "protected chats")
Closes #7112 (removing deprecated and unneeded APIs)