Fix Rails 7.2 deprecation warning for ActiveRecord::Base.connection

[benaitcheson]

Rails 7.2 soft-deprecated ActiveRecord::Base.connection in favour of lease_connection (rails/rails#51230). The bare connection calls in the reservation code paths trigger this warning on every job reservation:

DEPRECATION WARNING: Called deprecated `ActiveRecord::Base.connection` method.

Either use `with_connection` or `lease_connection`.

This adds a reservation_connection helper that uses lease_connection where available and falls back to connection on older Rails versions, keeping compatibility with the full supported ActiveRecord range. Same approach as #218 and #226 for earlier deprecations.

lease_connection returns the same leased-per-thread connection that connection did, so behaviour is unchanged.

[benaitcheson]

@danielmorrison / @albus522 if you have a moment, could I please get a review on this one?