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/templates/matrix-delegation-nginx.deployment.yaml

@@ -20,7 +20,7 @@ spec:
         {{- toYaml .Values.matrix.delegation.securityContext.pod | nindent 8 }}
       containers:
         - name: {{ .Release.Name }}-{{ .Chart.Name }}-delegation-nginx
-          image: "nginx:1.28.3"
+          image: "{{ .Values.matrix.delegation.image.repository }}:{{ .Values.matrix.delegation.image.tag }}"
           securityContext:
             {{- toYaml .Values.matrix.delegation.securityContext.container | nindent 12 }}
           ports: