Add minor updates for Python version to docs (#3446)

Add minor updates for Python version to docs.

### Description

Add minor updates for Python version to docs.

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Quick tests passed locally by running `./runtest.sh`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated.

Author: nvkevlu

docs/quickstart.rst

@@ -18,7 +18,7 @@ Supported Operating Systems
 Python Version
 --------------
 
-NVIDIA FLARE requires Python 3.8+.
+NVIDIA FLARE requires Python 3.9+.
 
 Install NVIDIA FLARE in a virtual environment
 ---------------------------------------------

docs/user_guide/federated_xgboost/secure_xgboost_user_guide.rst

@@ -40,6 +40,10 @@ XGBoost 2.2 or above, which can be installed from the binary build using this co
 
     pip install https://s3-us-west-2.amazonaws.com/xgboost-nightly-builds/federated-secure/xgboost-2.2.0.dev0%2B4601688195708f7c31fcceeb0e0ac735e7311e61-py3-none-manylinux_2_28_x86_64.whl
 
+.. note::
+
+   The xgboost build environment may depend on specific numpy versions that require Python < 3.12.
+
 or in case you need to get the most current build of XGBoost,
 
 .. code-block:: bash