Update logger.warn to logger.warning

Bug: 305035810
Change-Id: Ic2b35d5c3cbe92480c24da612f29382f5d26d4aa
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/389414
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>

subcmds/rebase.py

@@ -113,7 +113,7 @@ branch but need to incorporate new upstream changes "underneath" them.
             )
 
             if len(args) == 1:
-                logger.warn(
+                logger.warning(
                     "note: project %s is mapped to more than one path", args[0]
                 )