add upgrade configuration references documents

Author: Veki301

src/how-to/upgrade/05-config-reference.md

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+# Configuration Reference
+
+## Overview
+
+These references outline configuration changes between `wire-server` versions.
+It is intended for users who:
+
+- Maintain custom deployment templates
+- Use non-standard `wire-server-deploy` Ansible/Helm deployment
+- Need to know exact configuration changes to adapt their setups
+
+## Categories
+
+- Mandatory (breaking) changes - configuration changes that **must** be applied or services will fail to start
+- Optional changes - new features, enhancements or monitoring/logging options
+- Deprecated - configuration settings that can be removed or omitted, have no impact on the upgrade
+
+## How to use
+
+### Identify your versions
+
+- Current version you are running
+- Target version you want to upgrade to
+
+### Review all intermediate versions
+
+Configuration changes are cumulative. Review each version reference for mandatory changes.
+
+### Apply config to your templates
+
+Apply each configuration change to your templates as applicable to your model.
+
+## References
+
+- [Wire Server 5.24.0](config-references/wire-server-5.24.0.md)
+
+## Contributing
+
+If you find configuration changes not documented here, please report them to us directly or through a GitHub issue in our [documentation repository](https://github.com/wireapp/wire-docs). Include version numbers, configuration section and exact change needed.

src/how-to/upgrade/config-references/wire-server-5.24.0.md

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+# Wire-Server 5.24.0 release 
+
+## Mandatory (breaking) changes
+
+### `background-worker`
+
+```yaml
+background-worker:
+  config:
+    federationDomain: "example.com" # must match federation domain used for the instance in other services (brig etc.)
+    cassandraBrig:
+      host: your-cassandra-host-or-service # same as your current cassandra.host value
+    cassandraGalley:
+      host: your-cassandra-host-or-service # same as your current cassandra.host value
+```
+
+#### Conversation Data Migration
+
+The following configuration is only mandatory if you decide to migrate conversation data to PostgreSQL at this stage.
+
+Starting this release, migrating conversation data to PostgreSQL from Cassandra is possible. This is only required for channel search and channel management from Team Settings. Follow [this document](../../../developer/reference/config-options.md#using-postgresql-for-storing-conversation-data) for the steps and configuration required.
+
+If you do so, the following configurations are for `background-worker` are required.
+
+```yaml
+background-worker:
+  config:
+    postgresql:
+      host: your-postgresql-host-or-service
+```
+
+And for secrets:
+
+```yaml
+background-worker:
+  secrets:
+    pgPassword: "your-postgresql-password"
+```
+
+### `gundeck`
+
+Upstream Helm chart for `redis-ephemeral` has been replaced. New Redis service hostname has been changed from `{{ .Release.Name }}-master` to `{{ .Release.Name }}`. Verify your Redis service name with:
+
+```bash
+kubectl get svc | grep redis
+```
+
+Then set accordingly:
+
+```yaml
+gundeck:
+  config:
+    redis:
+      host: "your-redis-service"
+```
+
+## Optional changes
+
+### `background-worker`
+
+New settings, change only if required. The following are defaults as they come in charts
+
+```yaml
+background-worker:
+  postgresql:
+    host: postgresql # This one is already referenced in the mandatory category
+    port: "5432"
+    user: wire-server
+    dbname: wire-server
+  # Background jobs consumer configuration
+  backgroundJobs:
+    # Maximum number of in-flight jobs per process
+    concurrency: 8
+    # Per-attempt timeout in seconds
+    jobTimeout: 60s
+    # Total attempts, including the first try
+    maxAttempts: 3
+  postgresqlPool:
+    size: 5
+    acquisitionTimeout: 10s
+    agingTimeout: 1d
+    idlenessTimeout: 10m
+```