Privacy sandbox page: OWNERS update

Adding rainhard@ to OWNERS to continue the work on the privacy sandbox
page.

Bug: 1152336
Change-Id: Ic2dcf8feb24b44445e1b5ec63081af6ab126603a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2589976Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: Rainhard Findling <rainhard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837012}

chrome/browser/resources/settings/privacy_sandbox/OWNERS

+# Chrome Privacy Team
+rainhard@chromium.org
 sauski@google.com