Bump marshmallow from 3.21.1 to 3.21.2 in /backend

Bumps [marshmallow](https://github.com/marshmallow-code/marshmallow) from 3.21.1 to 3.21.2.
- [Changelog](https://github.com/marshmallow-code/marshmallow/blob/dev/CHANGELOG.rst)
- [Commits](marshmallow-code/marshmallow@3.21.1...3.21.2)

---
updated-dependencies:
- dependency-name: marshmallow
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

backend/requirements.txt

@@ -11,7 +11,7 @@ flask-jwt-extended==4.6.0
 flask-limiter==3.5.1
 flask-login==0.6.3
 flask-marshmallow==1.2.1
-marshmallow==3.21.1
+marshmallow==3.21.2
 marshmallow_sqlalchemy==1.0.0
 opsgenie-sdk==2.1.5
 pdpyras==5.2.0