Update CHANGES.md with Python prism note.

[lostluck]

Release branch equivalent to #33262 which updates the blog as well.

The note has already been added to the github release blurb: https://github.com/apache/beam/releases/tag/v2.61.0

Back updating the CHANGES for something that made the 2.61 release, but wasn't noted in the CHANGES

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Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

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[lostluck]

R: @damccorm

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