gclient: print use_relative_hooks deprecation on stderr.
gclient: print use_relative_hooks deprecation on stderr.

Printing to stdout breaks tools that parse the output of running
gclient.

TBR=ehmaldonado@chromium.org
Bug: chromium:1107325
Bug: chromium:1108301
Change-Id: I77f6eef9bdffd0534efb0b919304dab5b3df8c14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2312678
Commit-Queue: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Auto-Submit: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Reviewed-by: Michael Achenbach <76cc7f10de6bc994d83c4cb09072e4590b82b709@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools