Add key to ~/vue_merge_request_widget/components/widget/action_buttons.vue rendering of tertiary buttons

Follow-up from !191470 (comment 2559428868)

Background

Not all usages of ~/vue_merge_request_widget/components/widget/action_buttons.vue have an id key for the tertiaryButtons prop. This was causing problems in Vue 3 tests because we were using the index as the key.

Proposal

1. Add a validator to tertiaryButtons prop that requires a key attribute
2. Use key attribute for key in the template
3. Update all usages of ~/vue_merge_request_widget/components/widget/action_buttons.vue to have this key attribute