Bump Python version requirement from 3.11 to 3.14

Update all references to Python 3.11 to use Python 3.14 to match
the pyproject.toml requires-python = ">=3.14" setting:

- bin/lock_pkgs.sh: uv venv --python 3.14
- .github/workflows/test.yml: python matrix and PDM version
- .github/workflows/pip.yml: PYTHON_VERSION env var
- Dockerfile: comment and example FROM line
- Issue templates: example version output

bin/lock_pkgs.sh

@@ -45,7 +45,7 @@ echo
 echo
 
 echo "[+] Generating dev & prod requirements.txt & pdm.lock from pyproject.toml..."
-uv venv --allow-existing --python 3.11
+uv venv --allow-existing --python 3.14
 source .venv/bin/activate
 echo
 echo "pyproject.toml:    archivebox $(grep 'version = ' pyproject.toml | head -n 1 | awk '{print $3}' | jq -r)"