fix: bump jupyter-server from 1.16.0 to 2.20.0 in /packages/bigframes

Bumps [jupyter-server](https://github.com/jupyter-server/jupyter_server) from 1.16.0 to 2.20.0.
- [Release notes](https://github.com/jupyter-server/jupyter_server/releases)
- [Changelog](https://github.com/jupyter-server/jupyter_server/blob/main/CHANGELOG.md)
- [Commits](jupyter-server/jupyter_server@v1.16.0...v2.20.0)

---
updated-dependencies:
- dependency-name: jupyter-server
  dependency-version: 2.20.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

packages/bigframes/testing/constraints-3.11.txt

@@ -252,7 +252,7 @@ jsonschema-specifications==2025.4.1
 jupyter-client==6.1.12
 jupyter-console==6.1.0
 jupyter-leaflet==0.20.0
-jupyter-server==1.16.0
+jupyter-server==2.20.0
 jupyter_core==5.8.1
 jupyterlab_pygments==0.3.0
 jupyterlab_widgets==3.0.15