Design UX for migrating databases to KeePassX 2

The database format of KeePassX 2 is incompatible with KeePassX < 2 and the file extension is now kdbx instead od kbd. You can migrate a database to the new format but you cannot migrate back. We should make a plan for our users to migrate their database to the new format.

Related issues

Related to #12375 (closed)
Blocked by #12369 (closed)
Blocks #11917 (closed)

Original created by @sajolida on 12368 (Redmine)

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