bot: Update snsdemo to release-2026-01-21 (#7700)

# Motivation
We would like to keep the testing environment, provided by snsdemo, up
to date.

# Changes
* Updated `snsdemo` version in `config.json`.
* Ensured that the `dfx` version in `dfx.json` matches `snsdemo`.

# Tests
CI should pass.

Co-authored-by: gix-bot <[email protected]>

Author: pr-automation-bot-public[bot]

config.json

@@ -116,7 +116,7 @@
         "DIDC_VERSION": "didc 0.4.0",
         "POCKETIC_VERSION": "3.0.1",
         "CARGO_SORT_VERSION": "1.0.9",
-        "SNSDEMO_RELEASE": "release-2026-01-14",
+        "SNSDEMO_RELEASE": "release-2026-01-21",
         "IC_COMMIT_FOR_PROPOSALS": "release-2026-01-22_03-33-base",
         "IC_COMMIT_FOR_SNS_AGGREGATOR": "release-2026-01-22_03-33-base"
       },