[owners] Add leszeks@ as snapshot owner

Also remove yangguo@ at his behest.

Change-Id: Ief4176663ab01ff7b1d75aa8d6907c96ab519310
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2421822Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70057}

src/snapshot/OWNERS

 delphick@chromium.org
 jgruber@chromium.org
+leszeks@chromium.org
 verwaest@chromium.org
-yangguo@chromium.org
 
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