Deprecate Vue 2 only rules

[FloEdelmann]

These rules are currently in configs for Vue 2 (in all of essential, strongly-recommended and recommended), but not in configs for Vue 3:

• vue/no-custom-modifiers-on-v-model
  → could be useful as an uncategorized rule for Vue 3 to disallow custom modifiers, so let's not deprecate it
• vue/no-multiple-template-root
  → could be useful as an uncategorized rule for Vue 3 to prevent attribute inheritance problems, so let's not deprecate it
• vue/no-v-for-template-key
  → makes no sense for Vue 3, so deprecate it (see Vue 3 Migration Guide: key Attribute)
• vue/no-v-model-argument
  → makes no sense for Vue 3, so deprecate it (see Vue 3 Migration Guide: v-model)
• vue/valid-model-definition
  → makes no sense for Vue 3, so deprecate it (see Vue 3 Migration Guide: v-model)
• vue/valid-v-bind-sync
  → makes no sense for Vue 3, so deprecate it (see Vue 3 Migration Guide: v-model)

I didn't find any uncategorized rules that would only be useful for Vue 2.

Note

These rules will continue to work fine in the upcoming major version v10, but no longer receive much maintenance work, and we'll remove them in v11.

Thus, deprecating them is not technically a breaking change, but I think it will receive more attention if we do it in a major version.

[waynzh]

LGTM!