Really remove package:intl dependency

[mosuem]

Description

Follow up to #682 ...

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I have added sample code updates to the changelog.
• I updated/added relevant documentation (doc comments with ///).

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[gemini-code-assist]

Code Review

This pull request successfully removes the intl dependency from both examples/custom_backend/pubspec.yaml and packages/json_schema_builder/pubspec.yaml, aligning with the pull request's stated goal. Additionally, json_schema_builder has been correctly moved to dev_dependencies in the custom_backend example, and pubspec.lock has been updated to reflect these dependency changes and minor version bumps for testing packages.

[gemini-code-assist]

The lints package, which provides a recommended set of linting rules, has been removed from dev_dependencies. While not directly impacting functionality, removing this without an alternative linting setup could lead to a gradual degradation of code style and quality over time. It's generally advisable to maintain consistent linting practices to ensure code maintainability.