From log:
Aborting downgrade from (at least) 5.7 to 5.6. If are sure you want to downgrade to 5.6, remove the file /var/lib/mysql/debian-*.flag and try installing again.
From log:
Aborting downgrade from (at least) 5.7 to 5.6. mysql/debian- *.flag and try installing again.
If are sure you want to downgrade to 5.6, remove the file
/var/lib/