Updated Mtk-su to r21 (By Diplomatic@Xda)

Changelog:

• Support for the latest version of Magisk v20+
• Mtk-su(Diplomatic@Xda) updated to version r21, changes:
  • Add support for more devices
  • Fix seccomp on 3.18 arm kernels
• Modules pass minimum version check
• Faster and more efficient