https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46721 introduced a
new rubocop that enforced periods at the end of GraphQL description
strings. Existing offenses were added to `.rubocop_manual_todo.yml`.

This change fixes offenses in `app/graphql/mutations` and subdirectories
and removes the exclusion of them from `.rubocop_manual_todo.yml`.

The changes were generated using the following autocorrect:

    bundle exec rubocop app/graphql/mutations \
      --auto-correct \
      --only Graphql/Descriptions

https://gitlab.com/gitlab-org/gitlab/-/issues/292426