PIPE-2922: update Pixi CASA env matrix and tighten version bounds
- Bump casatasks/casatools minimum from >=6.6.6 to >=6.7.4.2
- Remove obsolete casa671-py312 feature/environment (no longer compatible)
- Rename casa675 comment from "dev/snapshot" to "latest release branch"
- Add casa676-py312 and casa676-py313 environments for CASA 6.7.6 (master branch)
- Switch default + docs environments from casa675 to casa674 (PL2026-CASA branch)
- Drop no-op solo solve-groups from named environments; keep only the
default/docs pairing which share a meaningful solve-group
