chore: update Redis database assignments link in migration guide

[ovasdi]

Replaced links to staging and production Redis database assignments with a link to the Notion table.

Once approved TODO:

• remove staging shared-redis-db-assignments configmap
• remove production shared-redis-db-assignments configmap
• update retiring systems playbook to point to the same database

[ovasdi]

# stg cleanup
kubectl --context staging delete configmap shared-redis-db-assignments
configmap "shared-redis-db-assignments" deleted

# prd cleanup
kubectl --context production delete configmap shared-redis-db-assignments
configmap "shared-redis-db-assignments" deleted