Preserve extension data in worksheets
- Require lxml<5 to avoid BC-breaking issues in 5.0.0.
- Clean up openpyxl.xml.functions to ensure lxml's iterparse is used when available.
- Fix #1745 by maintaining all generic XML elements under extlst.
This instance will be upgraded to Heptapod 17.11.0rc1 on 2025-04-24 between 17:00 and 18:00 UTC+2