Make delegation nginx image configurable in matrix-synapse

The delegation nginx image was hardcoded as nginx:1.28.3 in the
deployment template. It is now rendered from the new values
matrix.delegation.image.repository / matrix.delegation.image.tag,
defaulting to nginx:1.30.4 (current stable line, tag verified against
Docker Hub). README documents that auxiliary/sidecar images are pinned
via values and reviewed with every app-version update round.

Chart version: 4.1.1+upv1.159.0 -> 4.2.0+upv1.159.0 (minor: additive
values change).

matrix-synapse/Chart.yaml

@@ -2,7 +2,7 @@ apiVersion: v2
 name: matrix-synapse
 description: A Helm chart to deploy a Matrix Synapse server on Kubernetes
 type: application
-version: 4.1.1+upv1.159.0
+version: 4.2.0+upv1.159.0
 appVersion: "v1.159.0"
 maintainers:
   - name: jklein