fix(dashboard): Change class name on last Droppable in a column

[rtexelm]

SUMMARY

This is meant to allow resizing of each item in a column. No longer will the last child be bereft of resizing capabilities.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

B: A:

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• [x] Has associated issue: Fixes #28209
• [ ] Required feature flags:
• [ ] Changes UI
• [ ] Includes DB Migration (follow approval process in SIP-59)
  • [ ] Migration is atomic, supports rollback & is backwards-compatible
  • [ ] Confirm DB migration upgrade and downgrade tested
  • [ ] Runtime estimates and downtime expectations provided
• [ ] Introduces new feature or API
• [ ] Removes existing feature or API