Use kebab-case user list events

What does this MR do and why?

Updates the User Lists add-user modal custom event from addUsers to add-users, together with its parent listener and specs.

This addresses one small slice of the vue/custom-event-name-casing cleanup and removes the fixed files from the ESLint todo lists.

References

Related to: #583558

Screenshots or screen recordings

Not applicable. This is an internal Vue custom event casing change with no visible UI change.

How to set up and validate locally

1. Run Prettier on staged files:

   yarn run lint:prettier:staged:fix

2. Run ESLint for the changed area:

   node scripts/frontend/eslint.js --no-warn-ignored --no-error-on-unmatched-pattern --format gitlab .eslint_todo/vue-custom-event-name-casing.mjs .eslint_todo/vue-v-on-event-hyphenation.mjs app/assets/javascripts/user_lists/components

3. Run targeted Jest specs:

   yarn jest spec/frontend/user_lists/components/add_user_modal_spec.js spec/frontend/user_lists/components/user_list_spec.js

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #583558