Update the CI to use the minimum Android version supported by Flutter (#262)

Author: brianquinlan

.github/workflows/io_file.yml

@@ -115,7 +115,7 @@ jobs:
       - name: Run tests
         uses: reactivecircus/android-emulator-runner@1dcd0090116d15e7c562f8db72807de5e036a4ed
         with:
-          api-level: 21
+          api-level: 24
           disable-animations: true
           arch: x86_64
           target: default