[guix] Packages ogs-serial and ogs-petsc are now in official guix.

Packages ogs-serial and ogs-petsc are now in official guix. ogs-mkl and ogs-petsc-mkl packages are now defined in scripts/guix/packages and with these the custom guix channel (ogs-guix) is now obsolete.

Related:

• https://codeberg.org/guix/guix/pulls/851
• https://codeberg.org/guix/guix/pulls/2631

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented?