do not change db schema in an incompatible way (#5254)

PR #5099 removed some columns in the database that were actually in use.

usually, to not worsen UX unnecessarily
(releases take time - in between, "Add Second Device", "Backup" etc.
would fail), we try to avoid such schema changes (checking for
db-version would avoid import etc. but would still worse UX),
see discussion at #2294.

these are the errors, the user will be confronted with otherwise:

<img width=400
src=https://github.com/deltachat/deltachat-core-rust/assets/9800740/e3f0fd6e-a7a9-43f6-9023-0ae003985425>

it is not great to maintain the old columns, but well :)

as no official releases with newer cores are rolled out yet, i think, it
is fine to change the "107" migration
and not copy things a second time in a newer migration.

(this issue happens to me during testing, and is probably also the issue
reported by @lk108 for ubuntu-touch)

src/key.rs

@@ -294,11 +294,17 @@ pub async fn store_self_keypair(
                 KeyPairUse::ReadOnly => false,
             };
 
+            // `addr` and `is_default` written for compatibility with older versions,
+            // until new cores are rolled out everywhere.
+            // otherwise "add second device" or "backup" may break.
+            // moreover, this allows downgrades to the previous version.
+            // writing of `addr` and `is_default` can be removed ~ 2024-08
+            let addr = keypair.addr.to_string();
             transaction
                 .execute(
-                    "INSERT OR REPLACE INTO keypairs (public_key, private_key)
-                     VALUES (?,?)",
-                    (&public_key, &secret_key),
+                    "INSERT OR REPLACE INTO keypairs (public_key, private_key, addr, is_default)
+                     VALUES (?,?,?,?)",
+                    (&public_key, &secret_key, addr, is_default),
                 )
                 .context("Failed to insert keypair")?;