fix: packages/rest-api/package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-BODYPARSER-7926860
- https://snyk.io/vuln/SNYK-JS-EXPRESS-7926867
- https://snyk.io/vuln/SNYK-JS-SEND-7926862
- https://snyk.io/vuln/SNYK-JS-SERVESTATIC-7926865

packages/rest-api/package.json

@@ -23,7 +23,7 @@
     "@synapsecns/sdk-router": "^0.11.1",
     "bignumber": "^1.1.0",
     "ethers": "5.7.2",
-    "express": "^4.18.2",
+    "express": "^4.21.0",
     "lodash": "^4.17.21",
     "supertest": "^6.3.3"
   },