[roll] Roll fuchsia [starnix][range_map] Replace iter_starting_at with find_at_or_after

All the clients of this function just want to find a single mapping.
This formulation avoids the subtle distinction between this method and
how `range` operates.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1264704
Original-Revision: ad954b72e7228f5712e6106ce72106b08a72d284
GitOrigin-RevId: b4eeaa912a635858ffaf36b49a9fe8096bb968fe
Change-Id: I19ceac0b1f77d3ba4749441fc5ece975216144bb
diff --git a/stem b/stem
index 30d1817..a61d99d 100644
--- a/stem
+++ b/stem
@@ -8,7 +8,7 @@
              root=""
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="71f49c5d89036163c2281d2c62556482955a5550"
+             revision="ad954b72e7228f5712e6106ce72106b08a72d284"
              />
   </imports>
 </manifest>