chore(deps): bump plaid-python from 18.3.0 to 18.4.0 in /backend

Bumps [plaid-python](https://github.com/plaid/plaid-python) from 18.3.0 to 18.4.0.
- [Release notes](https://github.com/plaid/plaid-python/releases)
- [Changelog](https://github.com/plaid/plaid-python/blob/master/CHANGELOG.md)
- [Commits](plaid/plaid-python@18.3.0...18.4.0)

---
updated-dependencies:
- dependency-name: plaid-python
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

backend/dependencies/requirements.txt

@@ -1,7 +1,7 @@
 dynamodb-encryption-sdk==3.2.0
 mypy-boto3-dynamodb==1.34.0
 mypy-boto3-sqs==1.34.0
-plaid-python==18.3.0
+plaid-python==18.4.0
 python-jose[cryptography]==3.3.0
 requests==2.31.0
 urllib3<2