Pin seed-fu to 2.3.6 since 2.3.7 is broken and add a fixture file that uses seed-fu in the test env

What does this MR do?

This pins seed-fu to 2.3.6 since 2.3.7 is broken (see #40690 (closed)) and add a fixture file that uses seed-fu in the test env so that a broken version of seed-fu will be actually detected in the CI next time! 🤦

Are there points in the code the reviewer needs to double check?

I've just duplicated the admin seed file from the development env. 🤷

Why was this MR needed?

Because seeding was broken!

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #40690 (closed)