fix: explain replacement for VListItemAvatar/Icon

[yutak23]

For VListItemAvatar and VListItemIcon, i have changed to custom so that the substitution method is clear, and the message is what is written in the official Vuetify3 reference.

v-list-item-icon and v-list-item-avatar have been removed, use v-list-item with icon or avatar props, or put an icon or avatar in the append or prepend slot.

cf. https://vuetifyjs.com/en/getting-started/upgrade-guide/#v-list

This will make it easier for users of VListItemAvatar and VListItemIcon to migrate to Vuetify3.

[yutak23]

@KaelWD

I would appreciate it if you could review and respond to the content of the PR.