chore: prepare genui 0.10.3 release

[jacobsimionato]

Description

Bumps package:genui to 0.10.3 and creates the 0.10.3 changelog section for the PluralizeFunction fix merged in #1027.

0.10.2 was previously published to pub.dev on August 12 without #1027, so a patch bump is needed to release the change to pub.dev.

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 ///).

[gemini-code-assist]

Code Review

This pull request updates the version of the genui package from 0.10.2 to 0.10.3 in pubspec.yaml and CHANGELOG.md, attributing the PluralizeFunction fix to the new version. I have no feedback to provide.