ci: ignore rubygems warning in the "Documentation" job

ci: ignore rubygems warning in the "Documentation" job

A recent update in the Linux VM images used by Azure Pipelines surfaced
a new problem in the "Documentation" job. Apparently, this warning
appears 396 times on `stderr` when running `make doc`:

/usr/lib/ruby/vendor_ruby/rubygems/defaults/operating_system.rb:10: warning: constant Gem::ConfigMap is deprecated

This problem was already reported to the `rubygems` project via
https://github.com/rubygems/rubygems/issues/3068.

As there is nothing Git can do about this warning, and as the
"Documentation" job reports this warning as a failure, let's just
silence it and move on.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>