gclient: update logging.warn to logging.warning.

logging.warn is deprecated.

Change-Id: I047f356b8eeced631edc51e3b846f0995f4ce95e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2124526
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
diff --git a/gclient_scm.py b/gclient_scm.py
index d97c7e8..c58af6c 100644
--- a/gclient_scm.py
+++ b/gclient_scm.py
@@ -885,8 +885,8 @@
     except NoUsableRevError as e:
       # If the DEPS entry's url and hash changed, try to update the origin.
       # See also http://crbug.com/520067.
-      logging.warn(
-          'Couldn\'t find usable revision, will retrying to update instead: %s',
+      logging.warning(
+          "Couldn't find usable revision, will retrying to update instead: %s",
           e.message)
       return self.update(options, [], file_list)