Adds various tasks/checks that are currently in flutter/plugins but not flutter/packages to .cirrus.yaml. Most of these are currently harmless no-ops since they skip any packages without parts relevant to the check. Two substantive changes:

• Enables Gradle dependabot to manage all of the example apps, as we have in flutter/plugins; this will help avoid the gradle versions in examples getting more and more outdated over time.
  • As part of this, removed the android/ directory from the pigeon Obj-C e2e test directory, which was presumably just created by default and left there. (I expect to remove the whole e2e directory once the new Pigeon integration tests are in place, but this is the minimal change for now.)
• Moves Linux platform tests to the light workload group, which is where they are in flutter/plugins (since they don't actually need heavy workload machines).

Part of flutter/flutter#113764

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.