Add support for Dart 3

[mtrezza]

New Feature / Enhancement Checklist

• [x] I am not disclosing a vulnerability.
• [x] I am not just asking a question.
• [x] I have searched through existing issues.

Current Limitation

To add official support for Dart 3 the CI needs to run tests using Dart 3. There is currently no Dart matrix and all tests run with Dart 2.19.x. In fact, it's not defined which Dart version should be used and the CI package may change the Dart version at any time.

Feature / Enhancement Description

Add a Dart (and Flutter) version matrix to the CI to test against Dart (and Flutter) versions that are officially supported, according to our support policy currently discussed in #872.

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