Enabling the legacy protocol for Dart 3.3 results in no completions #5001

Completion tests are failing on Dart 3.3 when using the legacy protocol due to the removal of completion.getSuggestions.

Rather than migrating to the new APIs, we should disable using the legacy protocol on Dart 3.3 onwards. LSP has been stable and the default for a long time and we've not been updating the legacy client here to keep up with any new features.

As part of removing this, we should set up a bot that runs the tests using Dart 3.2 to ensure we're still testing the legacy protocol for older SDKs (because otherwise no legacy tests would run).