Swap bundler dependabot reviewers (#125333)

Remove myself as a the CocoaPods gem [dependabot](https://github.blog/2020-06-01-keep-all-your-packages-up-to-date-with-dependabot/) reviewer and add @christopherfujino and @vashworth instead for Ruby gem bumps:
3a436f47f5/dev/ci/mac/Gemfile

This is low traffic (one review every few months): https://github.com/flutter/flutter/pulls?q=is%3Apr+dependabot+cocoapods+is%3Aclosed

Note the last minor version was ignored https://github.com/flutter/flutter/pull/122051 due to https://github.com/flutter/flutter/issues/122057.

We have pretty good plugins/CocoaPods test coverage, so if that infra issue were fixed we should feel pretty safe about bumping this if all the tests pass.

.github/dependabot.yml

@@ -8,7 +8,8 @@ updates:
     schedule:
       interval: "weekly"
     reviewers:
-      - "jmagman"
+      - "christopherfujino"
+      - "vashworth"
       - "keyonghan"
       - "yusuf-goog"
     labels: