Add self as bundler dependabot reviewer (#140081)

Take back bundler/gem reviewership from @vashworth (thanks for covering!)

.github/dependabot.yml

@@ -9,7 +9,7 @@ updates:
       interval: "weekly"
     reviewers:
       - "christopherfujino"
-      - "vashworth"
+      - "jmagman"
       - "keyonghan"
       - "yusuf-goog"
     labels: