Set top-level version to 5.21.0 for sequential registration

5.20.0 was the last registered top-level version; 5.21.0 was set in
Project.toml in a prior PR but never registered on General. SciML#475 then
bumped straight to 5.22.0, which AutoMerge rejected for skipping
5.21.0. Register the intermediate version first so the next compat
bump can register cleanly as 5.22.0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

Project.toml

@@ -1,6 +1,6 @@
 name = "BoundaryValueDiffEq"
 uuid = "764a87c0-6b3e-53db-9096-fe964310641d"
-version = "5.22.0"
+version = "5.21.0"
 
 [deps]
 ADTypes = "47edcb42-4c32-4615-8424-f2b9edc5f35b"