Improve get preview channel changes script (#3270)

- Improve reliability of finding release note lines
- Identify cases where release notes were accidentally omitted (no "N/A
line)
- Filter out N/As

Release Notes:

- N/A
c67f78c065Joseph T. Lyons committed on 11/8/2023, 4:55:52 PM· committed by GitHubparents0143fa2e6bda02
1 file changedLine totals unavailable