support-tool: Fix data collector list render issue

The data collector list items are not rendered correclty. It causes the
test failres in the attached bug. Use dom-repeat instead of iron-list to
fix the issue for now for the upcoming branch. I'll do more
investigation about the reason as follow-up.
No visible UI change after this change.

Bug: b:334841776
Change-Id: I8ccd62ab72856afca7d2a6683fcefe78c5558752
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5605732


Commit-Queue: Irem Uguz <iremuguz@google.com>
Reviewed-by: Andrei Nedoluzhko <nedol@google.com>
Cr-Commit-Position: refs/heads/main@{#1312633}