[llvm][Docs] Add new LLDB Python guidance to release notes (llvm#122719)

As decided in
https://discourse.llvm.org/t/rfc-lets-document-and-enforce-a-minimum-python-version-for-lldb/82731
and implemented by llvm#114807.

Author: DavidSpickett

llvm/docs/ReleaseNotes.md

@@ -379,6 +379,10 @@ Changes to the LLVM tools
 Changes to LLDB
 ---------------------------------
 
+* It is now recommended that LLDB be built with Python >= 3.8, but no changes
+  have been made to the supported Python versions. The next release, LLDB 21,
+  will require Python >= 3.8.
+
 * LLDB now supports inline diagnostics for the expression evaluator and command line parser.
 
   Old: