Bump IREE to 3.12.0rc20260421

IREE: 3.12.0rc20260416 -> 3.12.0rc20260421
Signed-off-by: iree-pr-automator[bot] <iree-github-actions-bot@google.com>

Author: iree-github-actions-bot

version.json

@@ -1,5 +1,5 @@
 {
   "package-version": "0.0.1.dev",
-  "iree-version": "3.12.0rc20260416",
+  "iree-version": "3.12.0rc20260421",
   "therock-version": "7.13.0a20260420"
 }