docs: note Python 3.9 boto3 split in v9.15 release notes

Annotate the boto3 1.42 -> 1.43 dependency line to reflect that 1.43
requires Python >=3.10, so 3.9 stays on the 1.42.* series.

Author: asheshv

docs/en_US/release_notes_9_15.rst

@@ -89,7 +89,7 @@ Non-breaking ``dependabot`` updates aggregated for v9.15.
 
 Python:
 
-  | ``boto3`` 1.42 -> 1.43
+  | ``boto3`` 1.42 -> 1.43 (``python_version > '3.9'``; Python 3.9 stays on 1.42)
   | ``cryptography`` 46.0 -> 47.0
   | ``psycopg`` 3.3.3 -> 3.3.4 (``python_version >= '3.10'``)
   | ``pycodestyle`` >=2.5.0 -> >=2.14.0