Update spack to v0.21

[jonasjucker]

Ben Cumming CSCS:

Hello Jonas.
Do you have plans to update the c2sm-spack configuration to use Spack v0.21 ?
I ask because v0.21 is the first version with full support for Grace-Hopper (i.e. correctly detecting the micro-architecture of Grace and setting the correct compiler flags)
2:20
On the new system, I will be building environments with spack v0.21. And the “spack database” used by v0.21 is not compatible with v0.20.
2:22
I think that it is possible to provide Grace-Hopper images with v0.20, but they images won’t be optimal.

[jonasjucker]

solved in #909