minetest: Handle upgrade from 5.3.0 to 5.6.1

Closes: #2134 (closed).

Tests:

1. In testing container, install Minetest and change the configuration.

2. Manually downgrade minetest-server and minetest-data to a slightly older version (5.5.0+dfsg+~1.9.0mt4+dfsg-1).

3. In /var/lib/dpkg/status, change the hash for /etc/minetest/minetest.conf.

4. Run "apt update".

   • minetest-server package is upgraded.
   • Configuration changes are kept.

Signed-off-by: James Valleroy jvalleroy@mailbox.org