Add autosubmit label to dependabot PRs (flutter#8101)

The [flutter/flutter](https://github.com/flutter/flutter/blob/09941e7b4781b81adeb7bce8b389e5b8b45325ca/.github/dependabot.yml#L19) and [flutter/engine](https://github.com/flutter/engine/blob/5f8e6e465584599bef234b5fea4320cf78041cc2/.github/dependabot.yml#L12-L13) repo put the `autosubmit` label on dependabot PRs so if all checks pass then no review is necessary and it will merge.  For example, flutter#156024 merged without any human intervention.

In packages this is rarer (due to CHANGELOG and version bumps), but still happens, for example flutter/packages#8097 bumps a `testImplementation` dependency and passed all checks.  

Add `autosubmit` to the packages repo for dependabot PRs so if all checks pass they just merge.  If checks don't pass, the bot will remove the label and we'll be back to manual review required.

Author: jmagman

.github/dependabot.yml

@@ -69,6 +69,8 @@ updates:
       prefix: "[gradle]"
     schedule:
       interval: "weekly"
+    labels:
+      - "autosubmit"
     open-pull-requests-limit: 10
     ignore:
       - dependency-name: "*"
@@ -101,6 +103,8 @@ updates:
     schedule:
       interval: "weekly"
     open-pull-requests-limit: 10
+    labels:
+      - "autosubmit"
     ignore:
       - dependency-name: "com.android.tools.build:gradle"
         update-types: ["version-update:semver-minor", "version-update:semver-patch"]