fix(validatable): revalidate on rules change

[jacekkarczmarczyk]

Description

Motivation and Context

fixes #4635

How Has This Been Tested?

visually

Markup:

<template>
<v-app>
  <v-content>
    <v-container>
      <v-text-field v-model="a" :rules="validation" />
      <v-text-field v-model="errorMessage" />
    </v-container>
  </v-content>
</v-app>
</template>

<script>
  export default {
    data: () => ({
      a: '',
      errorMessage: 'error',
    }),

    computed: {
      validation () {
        return [(message => v => !v || message)(this.errorMessage)]
      }
    }
  }
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes, dev for new features and breaking changes).
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have created a PR in the documentation with the necessary changes.

Other comments

Don't know how to reproduce the issue from the removed comment

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