Bump axios, postmark and @serverless/platform-client in /backend

Bumps [axios](https://github.com/axios/axios) to 1.7.3 and updates ancestor dependencies [axios](https://github.com/axios/axios), [postmark](https://github.com/ActiveCampaign/postmark.js) and @serverless/platform-client. These dependencies need to be updated together.


Updates `axios` from 0.21.4 to 1.7.3
- [Release notes](https://github.com/axios/axios/releases)
- [Changelog](https://github.com/axios/axios/blob/v1.x/CHANGELOG.md)
- [Commits](axios/axios@v0.21.4...v1.7.3)

Updates `postmark` from 3.0.15 to 4.0.4
- [Release notes](https://github.com/ActiveCampaign/postmark.js/releases)
- [Changelog](https://github.com/ActiveCampaign/postmark.js/blob/main/CHANGELOG.md)
- [Commits](ActiveCampaign/postmark.js@3.0.15...4.0.4)

Updates `@serverless/platform-client` from 4.4.0 to 4.5.1

---
updated-dependencies:
- dependency-name: axios
  dependency-type: indirect
- dependency-name: postmark
  dependency-type: direct:production
- dependency-name: "@serverless/platform-client"
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

backend/package-lock.json

@@ -75,7 +75,7 @@
     "openai": "^4.12.1",
     "pg": "^8.8.0",
     "pg-query-stream": "^4.3.0",
-    "postmark": "^3.0.15",
+    "postmark": "^4.0.4",
     "reflect-metadata": "^0.1.13",
     "serverless-http": "^3.2.0",
     "sharp": "^0.31.3",