[DPE-6965] Re-enable charm upgrade tests (new snap _daemon_ user) (#1031)

The charm has been merged and released, we can re-enable test.

Author: taurus-forever

tests/spread/test_upgrade_from_stable.py/task.yaml

@@ -5,6 +5,3 @@ execute: |
   tox run -e integration -- "tests/integration/$TEST_MODULE" --model testing --alluredir="$SPREAD_TASK/allure-results"
 artifacts:
   - allure-results
-systems:
-  - -ubuntu-24.04
-  - -ubuntu-24.04-arm