Fix migration

[jacquesbh]

Previous title: Add method findTypeForMigration to ProductAttributeRepository

Q	A
Branch?	master
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	fixes #11592
License	MIT

I don't know how to test these except by running the database migrations.
So…

I've added the repository in Core because I think it is where it belongs since
the Migrations are in the main App.

[lchrusciel]

The base of this pull-request was changed, you need fetch and reset your local branch
if you want to add new commits to this pull request. Reset before you pull, else commits
may become messed-up.

Unless you added new commits (to this branch) locally that you did not push yet,
execute git fetch origin && git reset "fix-migration" to update your local branch.

Feel free to ask for assistance when you get stuck 👍

[lchrusciel]

Thank you, Jacques! 🥇