devserver: Prefer .tar.zst over .tar.bz2 in TELEMETRY_DEPS

Previously, TELEMETRY_DEPS checked .tar.bz2 before .tar.zst while
bzip2 tarballs were still being published. Now that builds only publish
.tar.zst, check .tar.zst first and add TODO(b/580903568) to drop the
.tar.bz2 fallback once pre-R158 branches (including R156 LTS) reach
EOL.

BUG=b:580903568, b:563203733
TEST=None

Change-Id: I6bc80b103b9e3be1a34bda83850ccab5d22dd64f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/8706712
Commit-Queue: Yoshiki IGUCHI <yoshiki@chromium.org>
Tested-by: Yoshiki IGUCHI <yoshiki@chromium.org>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
Auto-Submit: Yoshiki IGUCHI <yoshiki@chromium.org>
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>