Chore: [AEA-0000] - Make API key a secret var in Bruno (#4602)

## Summary

- Routine Change

### Details

I have rotated the leaked API key

Author: wildjames

bruno/EPS_Proxygen/environments/dev.bru

@@ -1,8 +1,8 @@
 vars {
   host: internal-dev.api.service.nhs.uk
-  api_key: zH8EpaM1hoFfrBkHqKdBdu8bqdkQTYBs
 }
 vars:secret [
+  api_key,
   client_secret,
   ptl_prescription_signing_key,
   ptl_prescription_public_key