[Task]: Single source of truth for Flutter version

To update Flutter in Playground, currently the following needs to be updated:

## Project files that declare minimal required versions as we desire, one per Flutter project:

https://github.com/apache/beam/blob/d20d0b01c3c6bcde551420f36e13d794c930f1e2/playground/frontend/playground_components/pubspec.yaml#L25

https://github.com/apache/beam/blob/d20d0b01c3c6bcde551420f36e13d794c930f1e2/playground/frontend/playground_components_dev/pubspec.yaml#L25

https://github.com/apache/beam/blob/d20d0b01c3c6bcde551420f36e13d794c930f1e2/playground/frontend/pubspec.yaml#L25

https://github.com/apache/beam/blob/d20d0b01c3c6bcde551420f36e13d794c930f1e2/learning/tour-of-beam/frontend/pubspec.yaml#L27

## Generated project files with computed minimal versions given the package requirements, one per runnable Flutter app:

https://github.com/apache/beam/blob/d20d0b01c3c6bcde551420f36e13d794c930f1e2/playground/frontend/pubspec.lock#L1068

https://github.com/apache/beam/blob/d20d0b01c3c6bcde551420f36e13d794c930f1e2/learning/tour-of-beam/frontend/pubspec.lock#L1117

## Pinned versions for tools that work on our projects:

https://github.com/apache/beam/blob/d20d0b01c3c6bcde551420f36e13d794c930f1e2/playground/frontend/Dockerfile#L20

https://github.com/apache/beam/blob/d20d0b01c3c6bcde551420f36e13d794c930f1e2/.github/workflows/build_playground_frontend.yml#L40

Need a single place to define the version instead.

## CI

https://github.com/apache/beam/blob/fbeae980e93ea64fc8cc3ad074cbc8aebd157691/.github/workflows/playground_frontend_test.yml#L40

https://github.com/apache/beam/blob/fbeae980e93ea64fc8cc3ad074cbc8aebd157691/.github/workflows/tour_of_beam_frontend_test.yml#L42

### Issue Priority

Priority: 3

### Issue Component

Component: beam-playground