Use latest Flutter stable and refactor SDK version parameters

[joshuadeguzman]

SEMVER Update Type:

• [ ] Major
• [ ] Minor
• [ ] Patch

Description:

The docs for this orb is a bit outdated and might confused newcomers trying out this orb, for example, the version parameter issues.

I fixed the version requirement and make use of flutter_version wherever the Flutter SDK needs to be set.

Next, changed the usage examples with the updated parameters and missing checkout (causes cache key issues).

Last, I updated the default version to the latest stable Flutter SDK release, including the sample app working on the latest SDK.

Motivation:

• Closes #14
• Closes #13
• Closes #8

Checklist:

• [X] All new jobs, commands, executors, parameters have descriptions.
• [X] Usage Example version numbers have been updated.
• [X] Changelog has been updated.

[joshuadeguzman]

Hey guys, would you be able to take a look at this? @tadashi0713

[JalexChen]

hey @joshuadeguzman - thank you for putting this PR in. at first glance, this looks alright, but we are holding off on releasing any orb updates at the moment. will revisit this in a bit and let the job run

[JalexChen]

Closing this out as the changes made would need to be added with @joshuadeguzman accepting any PRs. Instead, a lot of the changes here are now in PR #23 so that he still gets contributor credit