devserver: deprecate latestbuild rpc

The provision_Cr50TOT test has been updated to use gs.py. The
latestbuild RPC can not be deprecated.

BUG=chromium:1092689
TEST=None

Cq-Depend: chromium:2341955
Change-Id: I6d6fc85f3e92aeb446001c4e7760bc25e141bf99
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2340558
Tested-by: Sanika Kulkarni <sanikak@chromium.org>
Auto-Submit: Sanika Kulkarni <sanikak@chromium.org>
Reviewed-by: Congbin Guo <guocb@chromium.org>
Commit-Queue: Sanika Kulkarni <sanikak@chromium.org>
diff --git a/devserver.py b/devserver.py
index 8a71e14..acd4cdf 100755
--- a/devserver.py
+++ b/devserver.py
@@ -825,6 +825,9 @@
           R19-1993.0.0-a1-b1480.
       An empty string if no latest could be found.
     """
+    if is_deprecated_server():
+      raise DeprecatedRPCError('latestbuild')
+
     if not kwargs:
       return _PrintDocStringAsHTML(self.latestbuild)