docs: Add changelog entry for Rich default callbacks feature

Added entry to CHANGELOG.md documenting the new feature that defaults to RichProgressBar and RichModelSummary when the rich package is available, with fallback to TQDM variants.

Author: littlebullGit

src/lightning/pytorch/CHANGELOG.md

@@ -17,6 +17,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - For cross-device local checkpoints, instruct users to install `fsspec>=2025.5.0` if unavailable ([#20780](https://github.com/Lightning-AI/pytorch-lightning/pull/20780))
 
+- Default to RichProgressBar and RichModelSummary if the rich package is available. Fallback to TQDMProgressBar and ModelSummary otherwise. ([#9580](https://github.com/Lightning-AI/pytorch-lightning/pull/9580))
+
 
 ### Changed