Debci::DB: drop compatibility with ActiveRecord < 7

Antonio Terceirorequested to merge
activerecord-7-only into master

Debian stable has 7.2 these days, so there is no point in supporting anything below that.

While at it, this also simplifies the code and removes indirection layers, such as the LEGACY_MIGRATION alias. Let's just use ActiveRecord the way it was meant to be used.

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