NPA-6501: bump spec version before release (#318)

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## 🏷️ Naming Conventions Reminder

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Author: ehallam

specification/validated-relationships-service-api.yaml

@@ -3,7 +3,7 @@
 openapi: "3.0.0"
 info:
   title: "Validated Relationships Service API"
-  version: "1.17.0"
+  version: "1.18.0"
   description: |
     ## Overview
     This service underpins the National Proxy Programme whose core objective is to allow Patients to be supported with