Lint against deprecated slot syntax

We should enable the following rules:

• vue/no-deprecated-slot-attribute
• vue/no-deprecated-scope-attribute
• vue/no-deprecated-slot-scope-attribute

This will help with our eventual migration to Vue 3.

Implementation plan

• Enable in gitlab-org/gitlab explicitly (see gitlab-org/gitlab!64604 (comment 608284676))
• Enable in gitlab-org/frontend/eslint-plugin (perhaps just extend plugin:vue/vue3-essential?)
• Consume new @gitlab/eslint-plugin in gitlab-org/gitlab, and remove explicit opt-in