use newer drone image

.drone.yml

@@ -24,7 +24,7 @@ pipeline:
       #- adb shell am instrument -w -e debug false -e class com.owncloud.android.datamodel.OCFileUnitTest com.owncloud.android.test/android.support.test.runner.AndroidJUnitRunner
 
     environment:
-      - ANDROID_TARGET=android-24
+      - ANDROID_TARGET=android-31
       - ANDROID_ABI=armeabi-v7a
       - LD_LIBRARY_PATH=/usr/lib/x86_64-linux-gnu/:/opt/android-sdk-linux/tools/lib64/gles_mesa/