deps: bump jupyter_server to 2.21.0 and anyio to 4.14.2

[katsugtgz]

Bumps two pins in requirements.txt to clear known advisories.

Changes:

• jupyter_server 2.20.0 -> 2.21.0 (GHSA-c3mw-737p-c7g2)
• anyio 4.14.0 -> 4.14.2 (GHSA-82r6-8w77-94w6, GHSA-3w57-8xmc-8v26)

Evidence:

• requirements.txt pinned jupyter_server==2.20.0 and anyio==4.14.0
• osv-scanner reported all three advisories against requirements.txt before this change

Validation:

• osv-scanner no longer reports these advisories for either package
• both versions resolve on PyPI; all runtime floors required by jupyter_server 2.21.0 (anyio>=3.1.0, jupyter-client>=7.4.4, jupyter-events>=0.11.0, etc.) are already satisfied by the pinned versions in this file
• repo has no CI or test suite; requirements.txt is the documented install path in README (pip install -r requirements.txt), verified the pins resolve with pip install --dry-run

Scope: requirements.txt pin update only.