docs: bump tutorials submodule to 21c4b9a

Pulls in the updated RNA enrichment vignettes (scverse/decoupler-tutorials#1):
fixes the network plot colorbar label ("Gene-level statistic" instead of
"Gene expression"), migrates pyDESeq2 to the formulaic `design` argument,
and regenerates warning-free outputs. Rebuilds the docs with the corrected
tutorials.

Closes #331

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: PauBadiaM