Bump flask from 3.1.2 to 3.1.3 in /gateway-api

Bumps [flask](https://github.com/pallets/flask) from 3.1.2 to 3.1.3.
- [Release notes](https://github.com/pallets/flask/releases)
- [Changelog](https://github.com/pallets/flask/blob/main/CHANGES.rst)
- [Commits](pallets/flask@3.1.2...3.1.3)

---
updated-dependencies:
- dependency-name: flask
  dependency-version: 3.1.3
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

gateway-api/poetry.lock

@@ -10,7 +10,7 @@ requires-python = ">3.13,<4.0.0"
 
 [tool.poetry.dependencies]
 clinical-data-common = { git = "https://github.com/NHSDigital/clinical-data-common.git", tag = "v0.1.0" }
-flask = "^3.1.2"
+flask = "^3.1.3"
 types-flask = "^1.1.6"
 requests = "^2.32.5"