[resultdb] Use task: prefix
[resultdb] Use task: prefix

Use task: prefix instead of swarm: for swarming tasks.

Change-Id: I8daafea6a2219b33791120e6cc16d6d3c391678d
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1980927
Reviewed-by: Jao-ke Chin-Lee <5de1a387cc854ff3d04fab3d66d4e77a58511771@chromium.org>
Reviewed-by: Chan Li <332b6c56634d0f63001046c664a2370006a02420@chromium.org>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go