ci: Add tests against the beta version of Flutter/Dart

[Nidal-Bakir]

New Pull Request Checklist

• [x] I am not disclosing a vulnerability.
• [x] I am creating this PR in reference to an issue.

Issue Description

Closes: #888

Approach

TODOs before merging

• [ ] Add tests
• [ ] Add changes to documentation (guides, repository pages, in-code descriptions)
• [ ] A changelog entry

[parse-github-assistant[bot]]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[Nidal-Bakir]

Rerun when #883 & #884 merged and released.

The beta job is failing because of the upper bound but the other jobs are failing due to an issue with the code coverage upload. I'm not sure why this is happening.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (2bd71f9) 39.13% compared to head (f287c7f) 39.13%.

:exclamation: Current head f287c7f differs from pull request most recent head 0c75b2c. Consider uploading reports for the commit 0c75b2c to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #889 +/- ## ======================================= Coverage 39.13% 39.13% ======================================= Files 56 56 Lines 3278 3278 ======================================= Hits 1283 1283 Misses 1995 1995 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.

[Nidal-Bakir]

Because parse_server_sdk_flutter depends on parse_server_sdk >=5.0.0 which requires SDK version >=2.18.0 <3.1.0, version solving failed.

The parse server Dart SDK 5.1.1 has not been published yet.

[mtrezza]

Yes, so once https://github.com/parse-community/Parse-SDK-Flutter/issues/905 is merged and 5.1.1 is published, we can merge this.

[Nidal-Bakir]

Yes