chore(python): bump Python runtime from 3.9 to 3.11

[DoDiODev]

Summary

Bump the Python runtime used across the project from 3.9 to 3.11. This is a
pure runtime bump: no library versions are changed and the existing Pydantic v1
code keeps working on 3.11.

Changes

File	Change
backend/Dockerfile	server image base python:3.9-slim-bookworm → python:3.11-slim-bookworm
backend/python/uv.sh	default uv venv python 3.9 → 3.11
backend/python/pydevlake/pyproject.toml	python = "~3.9" → ">=3.11,<3.12"
backend/python/plugins/azuredevops/pyproject.toml	python = "~3.9" → ">=3.11,<3.12"
backend/python/test/fakeplugin/pyproject.toml	python = "~3.9" → ">=3.11,<3.12"
.devcontainer/devcontainer.json	python feature 3.9 → 3.11
backend/python/DevelopmentSetup.md	docs now reference python3.11

Explicitly out of scope

• No library upgrades. sqlmodel, pydantic, pytest, inflect, fire,
  jsonpointer etc. stay unchanged. Pydantic v2 / SQLModel migration and the
  corresponding poetry.lock regeneration are handled in a follow-up PR to keep
  this change reviewable and independently revertable.
• No Node/frontend changes (the devcontainer Node feature stays at 18).

Testing

Automated (on Python 3.11.15):

• make build-pydevlake ✅ (resolves with pydantic 1.10.26, sqlmodel 0.0.8)
• make unit-test-python ✅ (azuredevops: 8 passed / 1 skipped, pydevlake: 10 passed)

Manual:

• Built the server image base stage from backend/Dockerfile → image reports
  Python 3.11.15, Python remote plugins build successfully.
• Started the azuredevops remote plugin and validated its plugin-info RPC
  handshake payload (name, extension, 12 subtasks, 14 migration scripts) on 3.11.
• Ran a real data-collection flow (Jira, git, SonarQube) end to end.

Known unrelated issue

devcontainer build currently fails to pull mcr.microsoft.com/devcontainers/go:1-1.26-bookworm
(.devcontainer/Dockerfile). This base-image reference is not touched by this PR
and fails identically on main; it is independent of the Python 3.11 change.

Rollback

Revert this single commit; it only changes version strings and has no schema or data impact.

[klesh]

Why are you bumping python runtime? Are you using any python plugin?

[DoDiODev]

@klesh We - the company for which I'm working - would like to use devlake. The precondition is that all dependencies and the used tooling is up to date. Therefore I'm working on this step by step. The reason behind this is to reduce security issues in the software used.
When I see it correctly there is only the Azure DevOps plugin using Python. In the first step this will not be used. But we have teams also using Azure.