Bump dart from `2d8257b` to `720e088` in /auto_submit (#2322)

Bumps dart from `2d8257b` to `720e088`.

---
updated-dependencies:
- dependency-name: dart
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
diff --git a/auto_submit/Dockerfile b/auto_submit/Dockerfile
index 39e4b3d..76e25b8 100644
--- a/auto_submit/Dockerfile
+++ b/auto_submit/Dockerfile
@@ -4,7 +4,7 @@
 
 
 # Dart Docker official images can be found here: https://hub.docker.com/_/dart
-FROM dart:beta@sha256:2d8257b23e5a247ea51d24b31689aaf5ec571a077cc0db3978bc67567b04d2ca
+FROM dart:beta@sha256:720e088ddac7cc033117e20578133e01d70183b43114889abc6fb4485b05b089
 
 WORKDIR /app