Update to version 7.6.21

[gothicx]

This version includes a number of significant updates, most notably the addition of ARM 64-bit support. ARM support is a great step forward given the rapid proliferation of ARM-based devices. We are especially grateful to Neocortix for helping make this possible!

The new client also includes updates that better detect and automatically configure supported GPUs.

https://foldingathome.org/2020/11/24/new-client-with-arm-support/