Fix schema after bulk mutations are added

[jxltom]

It looks the schema in #3838 is incorrect, the bulk mutation should return count. This PR fixes this issue.

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. The code is documented (docstrings, project documentation).
8. GraphQL schema and type definitions are up to date.
9. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #3843 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3843   +/-   ##
=======================================
  Coverage   90.79%   90.79%           
=======================================
  Files         263      263           
  Lines       14142    14142           
  Branches     1412     1412           
=======================================
  Hits        12840    12840           
  Misses        894      894           
  Partials      408      408

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