Implement dependabot groups in packages repo

[jmagman]

Dependabot now supports groups to update multiple packages in the same PR.
https://github.com/flutter/packages/blob/main/.github/dependabot.yml

Grouped version updates for Dependabot are generally available
Docs for groups

Ideally it would mean all the, say, gradle updates could happen in 1 PR, instead of 1 PR per package:
flutter/packages#6661
flutter/packages#6662
flutter/packages#6663
flutter/packages#6665
flutter/packages#6667
flutter/packages#6669

This would reduce the number of incoming commits to the packages repo, and reduce the review burden for keeping dependencies up-to-date.

[jmagman]

This will probably be easier for the packages repo when multi-directory becomes stable:
https://github.blog/changelog/2024-04-29-dependabot-multi-directory-configuration-public-beta-now-available/

[jmagman]

cc @stuartmorgan

[stuartmorgan]

/cc @reidbaker as well; the Android team should weigh in on which things it makes sense to batch. Some updates require other package-level changes, and in those cases batching could make things much harder to get landed.

[reidbaker]

FWIW I think Gradle updates can be batched as can Kotlin updates.
AGP updates require manual effort most of the time so while I would like to say they cant be batched the reality is it doesnt matter because we will have to manually update everything anyway.
I dont think we have dependabot setup for android versions but compileSdkVersion could likely also be batched.

@gmackall