Bump Android API level to 36 in CI (#231)

This is the version recommended by official Flutter docs:
https://docs.flutter.dev/platform-integration/android/setup#set-up-tooling

Author: emersion

.github/workflows/test.yml

@@ -195,7 +195,7 @@ jobs:
       - name: Run flutter test integration_test/webcrypto_test.dart -d android
         uses: reactivecircus/android-emulator-runner@v2
         with:
-          api-level: 28
+          api-level: 36
           working-directory: ./example
           script: flutter test integration_test/webcrypto_test.dart -d android
   linux-coverage: