Use dart latest version in pre-release tests

Author: Strift

.github/workflows/pre-release-tests.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        version: ["3.0.0"]
+        version: ['latest']
     name: integration-tests-against-rc (dart ${{ matrix.version }})
     steps:
       - name: Checkout