[roll] Roll fuchsia [fuchsia_runtime] UtcTime => UtcInstant

Aligning with the terminology introduced in the boot time RFC and the
recent refactors to use this terminology in the fuchsia_zircon Instant
structs.

Original-Bug: 361661898
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1128872
Original-Revision: 8cde64d837768012ea605d85cfa7b4c87e16fc2d
GitOrigin-RevId: 04c987f8465a769f2ef1007ade2b211fe1a45846
Change-Id: I1693f2af20ddf7e93a8ca722a01dd40ce184f142
diff --git a/stem b/stem
index 15c49d0..ef2992e 100644
--- a/stem
+++ b/stem
@@ -11,7 +11,7 @@
              githooks="integration/git-hooks"
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="53491311c7c132ab56134e8f7c7e2f7f97d4b067"/>
+             revision="8cde64d837768012ea605d85cfa7b4c87e16fc2d"/>
   </projects>
   <hooks>
     <hook name="install-environment"