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

.github/workflows/pip.yml

@@ -9,7 +9,7 @@ on:
       - 'v*'
 
 env:
-  PYTHON_VERSION: 3.11
+  PYTHON_VERSION: 3.14
 
 jobs:
   build:

.github/workflows/test.yml

@@ -15,7 +15,7 @@ jobs:
       matrix:
         os: [ubuntu-22.04]
         # os: [ubuntu-22.04, macos-latest, windows-latest]
-        python: [3.11]
+        python: [3.14]
 
     steps:
       - uses: actions/checkout@v4
@@ -38,7 +38,7 @@ jobs:
       - name: Setup PDM
         uses: pdm-project/setup-pdm@v3
         with:
-          python-version: '3.11'
+          python-version: '3.14'
           cache: true
 
       ### Install Python & JS Dependencies