Refactor dependabot configs.

[godofredoc]

This is to validate we can use app top level folders to config updates of inner pubspec files.

Bug: flutter/flutter#136983

Pre-launch Checklist

• I read the Effective Dart: Style recently, and have followed its advice.
• I signed the CLA.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.

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[devoncarew]

This is to validate we can use app top level folders to config updates of inner pubspec files.

Even with that, consider also adding: versioning-strategy: increase-if-necessary

The pub adaptor for dependabot will otherwise default to widen, which isn't what we want here (we want ^3.0.0, instead of something like >=2.1.0 <4.0.0). See also dependabot/dependabot-core#6786 and dependabot/dependabot-core#4979.

[godofredoc]

dependabot/dependabot-core#4979

Even with that, consider also adding: versioning-strategy: increase-if-necessary

The pub adaptor for dependabot will otherwise default to widen, which isn't what we want here (we want ^3.0.0, instead of something like >=2.1.0 <4.0.0). See also dependabot/dependabot-core#6786 and dependabot/dependabot-core#4979.

Done!

[domesticmouse]

This is to validate we can use app top level folders to config updates of inner pubspec files.

If we can update inner pubspec.yaml files, is there a reason not to use / as the target directory to update?