Eleven: gradle: bump compileSdkVersion to 31

This fixes:
  The minCompileSdk (31) specified in a
  dependency's AAR metadata (META-INF/com/android/build/gradle/aar-metadata.properties)
  is greater than this module's compileSdkVersion (android-30).

Change-Id: Ia0a97fdaa72862b39484114d32f55dfd83f18f0a
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>