Deprecate migration helper

[aldesantis]

Our migration helper was introduced for supporting the upgrade from Rails 4.2 to 5.x, which introduced support for migration versions.

Now that we don't support 4.2 anymore (the earliest Rails version supported by Solidus 2.9 is 5.1), we can deprecate the helper and simply use ActiveRecord::Migration#[] instead.

[stale]

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