fix: seeds (#2023)

Motivation
----------
Fix seeds which apparently broke in #1971.

How to test
-----------
1. `docker compose up db`
2. `bin/rails db:drop db:create db:migrate db:seed`
3. Does not fail anymore

db/seeds.rb

@@ -24,7 +24,8 @@
 admin = User.create_or_find_by!(email: 'redaktion@tactile.news', first_name: 'Dennis', last_name: 'Schröder', password: password,
                                 admin: true, otp_secret_key: otp_secret_key)
 user = User.create_or_find_by!(email: 'contact-person@example-organization.org', first_name: 'ConactFor', last_name: 'Organization',
-                               password: password, otp_secret_key: otp_secret_key, organization: organization)
+                               password: password, otp_secret_key: otp_secret_key)
+user.organizations << organization
 organization.update(contact_person: user)
 puts "Organization with name #{organization.name}"
 puts "Admin with email #{admin.email}"