update sliding-sync to v0.99.18

Author: shcherbak

charts/synapse/Chart.yaml

@@ -1,3 +1,3 @@
 ---
 name: synapse
-version: 0.1.4
+version: 0.1.5

charts/synapse/values.yaml

@@ -233,7 +233,7 @@ slidingSyncProxy:
   affinity: {}
   image:
     repository: ghcr.io/matrix-org/sliding-sync
-    tag: v0.99.15
+    tag: v0.99.18
   service:
     type: ClusterIP
     port: 80