Adapt recipe to Docs v6 (yhub collaboration server)

- the new yhub service needs new initialisation and RSA keys
- SOFT_MIGRATION on by default, add migrate_documents command
- back up the yhub database too
- document the v5 -> v6 upgrade in README and release/next

Following the [C4DT AI Policy](https://git.coopcloud.tech/c4dt/ai-policy/src/branch/main/AI-POLICY.md):

 #LLM_vibe
 #LLM_model Claude Opus 5.5

- [x] I take full responsibility for this code
- [x] I understand every line of this code
- [x] I tested this code thoroughly on my own computer / servers
- [x] None of the commit messages were created using LLMs
- [x] I will participate in any discussion on my own and will not use an LLM for the discussion

abra-entrypoint.sh

@@ -7,7 +7,6 @@ set -e
 [ -f /run/secrets/minio_ru ] && export AWS_S3_ACCESS_KEY_ID="$(cat /run/secrets/minio_ru)"
 [ -f /run/secrets/django_sp ] && export DJANGO_SUPERUSER_PASSWORD="$(cat /run/secrets/django_sp)"
 [ -f /run/secrets/oidc_rpcs ] && export OIDC_RP_CLIENT_SECRET="$(cat /run/secrets/oidc_rpcs)"
-[ -f /run/secrets/collab_ss ] && export COLLABORATION_SERVER_SECRET="$(cat /run/secrets/collab_ss)"
 [ -f /run/secrets/y_api_key ] && export Y_PROVIDER_API_KEY="$(cat /run/secrets/y_api_key)"
 [ -f /run/secrets/email_pass ] && export DJANGO_EMAIL_HOST_PASSWORD="$(cat /run/secrets/email_pass)"