change default job backend to Slurm + deprecate support for using GC3Pie as job backend

[boegel]

No description provided.

[gkaf89]

It seems that the current implementation of the Slurm backend does not support the experimental easystack feature.

When compiling easystacks compilation jobs are resubmitted without dependencies on previous entries of the easystack. Consider for instance the easystack file:

easyconfigs:
  - foss-2024.05.eb
  - HDF5-1.14.3-gompi-2023b.eb

Jobs for GCCcore are created for both foss-2024.05.eb and HDF5-1.14.3-gompi-2023b.eb, and there is no dependency that prevents the job for HDF5-1.14.3-gompi-2023b.eb to start after the job for foss-2024.05.eb.

I am mentioning the issue here because easystacks seem like a good use case for parallel compilation jobs.

[ocaisa]

@gkaf89 I looked into that in #4558 (comment) but I would need to find time to implement

[boegel]

@gkaf89 That's correct, the integration of easystack files and --job is far from optimal, but that's orthogonal to which job backend is being used (so better to continue that discussion in #4558, indeed)