Remove `confidence` field from `VulnerabilityCreate` GraphQL mutation

Why are we doing this work?

In %15.4 we actually deprecated the confidence field from VulnerabilityCreate GraphQL mutation but we didn't remove it.

Implementation plan

1. Remove confidence field from ee/app/graphql/mutations/vulnerabilities/create.rb
2. Add entry to doc/api/graphql/removed_items.md

Testing

• Since this is a removal of a field, safe to run e2e:package-and-test in the MR to make sure devopsgovern specs are green.