Change Venue Name field to textarea for multi-venue support

[dd32]

Summary

• Changes the Venue Name field from a single-line text input to a multi-line textarea
• Allows Campus Connect events with multiple venues to list them on separate lines
• Applied to both WordCamp and Campus Connect tracker venue field definitions

Test plan

• Edit a WordCamp or Campus Connect event in wp-admin
• Verify the Venue Name field is now a textarea (multi-line)
• Enter multiple venues on separate lines and save
• Verify the data is preserved correctly

Fixes WordPress#1604

Generated with Claude Code

[dd32]

Can you look at making it allow multiple venues, rather than using multi-line?
(Do this as a separate PR / alternate approach)