Add Pierre Dureau and Florent Torregrosa as co-maintainers for Asset library subsystem

**Problem/Motivation:**
Pierre Dureau ([pierredureau]( https://www.drupal.org/u/pierredureau → )) and Florent Torregrosa ([grimreaper]( https://www.drupal.org/u/grimreaper → )) have confirmed their interest in co-maintaining the Asset library subsystem.

**Proposed resolution:**
Add pierredureau and grimreaper as co-maintainers for the Asset library subsystem in `MAINTAINERS.txt`.

**Remaining tasks:**
- Commit the update to `MAINTAINERS.txt`
- Notify the Core Committers Slack channel when merged

No UI/API/Data model changes.