chore: Upgrade ubuntu CI runner versions (#239)

## Background

Ubuntu 20.04 has been deprecated in GitHub. The jobs using ubuntu-20.04
runner no longer runs. In the logs there is error:
```
This is a scheduled Ubuntu 20.04 retirement. Ubuntu 20.04 LTS runner will be removed on 2025-04-15. For more details, see actions/runner-images#11101
```

(e.g.
https://github.com/streamr-dev/streamr-docker-dev/actions/runs/14733830602/job/41354733466)

## Changes

Upgraded versions. Now using `ubuntu` 22 and 24 instead of 20 and 22.

Author: teogeb

.github/workflows/ci.yaml

@@ -41,8 +41,8 @@ jobs:
       fail-fast: false
       matrix:
         os:
-          - ubuntu-20.04
           - ubuntu-22.04
+          - ubuntu-24.04
     steps:
       - name: Check out the repo
         uses: actions/[email protected]