Hello,
You need to edit file /var/home/radius-admingroup-template-user/.ssh/known_hosts, as the prompt says, and delete line which contains "10.21.8.100"
Here is the procedure in shell, assuming You have privileges to edit this file:
1/ vi /var/home/radius-admingroup-template-user/.ssh/known_hosts
2/ look for 10.21.8.100
3/ delete the line containing 10.21.8.100
4/ save the file and exit vi editor
5/ repeat ssh attempt and accept the new key.
But You should be investigating the root cause first - why the SSH key changed? While this could happen after upgrade, or booting from alternate partition containing older/newer JUNOS version, the more sinister root causes such as someone remotely took over Your internet-exposed FW and implanted own backdoored firmware cannot be ruled out nowadays.
HTH
Thx
Alex