build(backend): Change base image to debian:13-slim w/ Python 3.13 (#10654)

- Resolves #10653

The objective is to move to a base image with fewer active
vulnerabilities. Hence the choice for `debian:13-slim` (0 high, 1
medium, 21 low severity), a huge improvement compared to our current
base image `python:3.11.10-slim-bookworm` (4 high, 11 medium, 15 low
severity).

### Changes 🏗️

- Change backend base image to `debian:13-slim`
  - Use Python 3.13
- Fix now-deprecated use of class property in `AppProcess` and
`BaseAppService`
- Expand backend CI matrix to run with Python 3.11 through 3.13
- Update Python version constraint in `pyproject.toml` to include Python
3.13

Also, unrelated:
- Update `autogpt-platform-backend` package version to `v0.6.22`, the
latest release

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] CI passes
  - [x] No new errors in deployment logs
  - [x] Everything seems to work normally in deployment

.github/workflows/platform-backend-ci.yml

@@ -32,7 +32,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.11"]
+        python-version: ["3.11", "3.12", "3.13"]
     runs-on: ubuntu-latest
 
     services: