build_packages: Switch to warn for deprecation message

BUG=b:216536410
TEST=manual (./build_packages --board ..) to verify output warning in
yellow

Change-Id: Ie5fe6b4346e164d44d6481e680ac283fa2c13003
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/3436858
Tested-by: Cindy Lin <xcl@google.com>
Auto-Submit: Cindy Lin <xcl@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Cindy Lin <xcl@google.com>
diff --git a/build_packages b/build_packages
index bc9523a..68dc80e 100755
--- a/build_packages
+++ b/build_packages
@@ -10,14 +10,11 @@
 restart_in_chroot_if_needed "$@"
 
 new_script="build_packages"
-cat <<EOF
-$0: This script is deprecated and will be removed.
-All users must migrate to ${new_script} in chromite/bin.
-You can simply change all references of $0 to \`${new_script}\`
-from \$PATH (in chromite/bin/).
-This old script will be removed by July 2022.
-If you have questions or found code that needs updating, please contact
-chromium-os-dev@, or file a bug at go/cros-build-bug.
-EOF
+warn "$0: This script is deprecated and will be removed."
+warn "All users must migrate to ${new_script} in chromite/bin."
+warn "You can simply change all references of $0 to \`${new_script}\`" \
+  "from \$PATH (in chromite/bin/)."
+warn "This old script will be removed by July 2022."
+warn "If you have questions or found code that needs updating, please" \
+  "contact chromium-os-dev@, or file a bug at go/cros-build-bug."
 exec "${CHROMITE_BIN}/${new_script}" "$@"
-