Migrate old Pi-hole owned dnsmasq config files into /etc/pihole/migration_backup_v6

[PromoFaux]

What does this PR aim to accomplish?:

This step from the bare metal install script has been missed so far in Docker, we simply run it on the first check if we detect that it is a v5->v6 upgrade

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[PromoFaux]

Files before this change:

And after