community/stress-ng: upgrade to 0.11.21

current version may suffer this probelm https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1875941 , cause kernel crash

added 2 commits

• 6df9f34e - 1 commit from branch alpine:master
• 958e167b - community/stress-ng: upgrade to 0.11.21

Compare with previous version

merged