Update repository to pass validate

[stuartmorgan-g]

This fixes everything that validate failed on locally when running flutter/packages#11784 locally against the repo. This is in preparation for enabling the repo tooling in this repo.

• Adds dependabot coverage for example apps (although it's currently not going to actually do anything; it depends whether we ever add Flutter examples for non-Flutter-specific packages)
• Adds autolabeling
• Adds the tool config
• Fixes repo-level README link mistakes
• Requires Dart 3.10 (N-2)
• Adds languages to code blocks in the README, and temporarily exempts the README from excerpts

Tooling will actually be enabled in a future PR, once the tooling updates are published to pub.dev.