Update Flutter CI Template to use the current Flutter image

What does this MR do and why?

Update the Flutter CI/CD template to use the latest docker image as at 22 May 2023.

Screenshots or screen recordings

Before	After
cirrusci/flutter:1.22.5	ghcr.io/cirruslabs/flutter:3.10.0
pub global activate dart_code_metrics	flutter pub global activate dart_code_metrics
`test --machine --coverage	tojunit -o report.xml`

How to set up and validate locally

1. Create a new flutter project
2. add .gitlab-ci.yml file with contents from template
3. Save, merge and push to GitLab to trigger CI

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.