fix: fix specification of many assignment mutations

### Changes

* changed argument `userIds: [ID]!` to `userIds: [ID!]!` on `assignUsersToTeam`
* changed argument `userIds: [ID]!` to `userIds: [ID!]!` on `removeUsersFromTeam`
* changed argument `userIds: [ID]!` to `userIds: [ID!]!` on `assignInstructorsToExercise`
* changed argument `userIds: [ID]!` to `userIds: [ID!]!` on `removeInstructorsFromExercise`
* changed argument `userIds: [ID]!` to `userIds: [ID!]!` on `addDefinitionAccess`
* changed argument `userIds: [ID]!` to `userIds: [ID!]!` on `removeDefinitionAccess`
* changed argument `userIds: [ID]!` to `userIds: [ID!]!` on `regenerateCredentials`
* changed argument `userIds: [ID]!` to `userIds: [ID!]!` on `deleteUsers`

Closes #473