chore: Bump jupyter-server-proxy from 3.1.0 to 3.2.3 in /umich-notebook

Bumps [jupyter-server-proxy](https://github.com/jupyterhub/jupyter-server-proxy) from 3.1.0 to 3.2.3.
- [Release notes](https://github.com/jupyterhub/jupyter-server-proxy/releases)
- [Changelog](https://github.com/jupyterhub/jupyter-server-proxy/blob/v3.2.3/CHANGELOG.md)
- [Commits](jupyterhub/jupyter-server-proxy@v3.1.0...v3.2.3)

---
updated-dependencies:
- dependency-name: jupyter-server-proxy
  dependency-type: direct:production
...

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

umich-notebook/requirements.txt

@@ -1,5 +1,5 @@
 git+https://github.com/jupyter/nbgrader.git@2256b7efd094c4e7863f76ef26d2149b893a4462
-jupyter-server-proxy==3.1.0
+jupyter-server-proxy==3.2.3
 nbgitpuller==0.10.2
 psycopg2-binary==2.9.1
 jupyter-resource-usage==0.6.0