Add enviroment validation at server startup phase (#3388)

### What problem does this PR solve?

1. Validate the Python version should >= 3.11
2. Download nltk data

### Type of change

- [x] New Feature (non-breaking change which adds functionality)
- [x] Refactoring

---------

Signed-off-by: jinhai <haijin.chn@gmail.com>
Co-authored-by: jinhai <haijin.chn@gmail.com>
Co-authored-by: Zhichang Yu <yuzhichang@gmail.com>

api/ragflow_server.py

@@ -22,6 +22,7 @@ import time
 import traceback
 from concurrent.futures import ThreadPoolExecutor
 
+import validation
 from werkzeug.serving import run_simple
 from api.apps import app
 from api.db.runtime_config import RuntimeConfig