devel: officially deprecate update without destination

When we introduce the develwarning, we did not had an official deprecation API
and infrastructure. We can now officially deprecate the old way with a version
deadline.

mercurial/merge.py

@@ -1442,7 +1442,7 b' def update(repo, node, branchmerge, forc'
             pas = [repo[ancestor]]
 
         if node is None:
-            repo.ui.develwarn('update with no target', 'oldapi')
+            repo.ui.deprecwarn('update with no target', '3.9')
             rev, _mark, _act = destutil.destupdate(repo)
             node = repo[rev].node()