fix(DataTable): allow removal of additional sortings

[KYash03]

Fixes #20985

What I Did: The issue was with must-sort and multi-sort enabled, sorted columns couldn't be removed, only toggled ascending/descending. I modified toggleSort so in multi-sort mode, columns sorting descending are removed if multiple sorts are active, but toggle back to ascending if they're the only sorted column (satisfying must-sort).

How to Verify: Click a column header to sort ascending, then again for descending. Add another sorted column to test multi-sort - when you click the descending column, it should get removed since there's multiple sorts. Try with just one sorted column too - clicking it should just flip the sort direction instead of removing it.

<template>
  <v-app>
    <v-container>
      <v-data-table
        :headers="headers"
        :items="desserts"
        :must-sort="true"
        :multi-sort="true"
      />
    </v-container>
  </v-app>
</template>

<script>
export default {
  name: 'Playground',
  setup() {
    const headers = [
      {
        title: 'Dessert (100g serving)',
        key: 'name',
      },
      {
        title: 'Calories',
        key: 'calories',
      },
      {
        title: 'Fat (g)',
        key: 'fat',
      },
      {
        title: 'Carbs (g)',
        key: 'carbs',
      },
      {
        title: 'Protein (g)',
        key: 'protein',
      },
    ]

    const desserts = [
      {
        name: 'Frozen Yogurt',
        calories: 159,
        fat: 6.0,
        carbs: 24,
        protein: 4.0,
      },
      {
        name: 'Ice cream sandwich',
        calories: 237,
        fat: 9.0,
        carbs: 37,
        protein: 4.3,
      },
      {
        name: 'Eclair',
        calories: 262,
        fat: 16.0,
        carbs: 23,
        protein: 6.0,
      },
      {
        name: 'Cupcake',
        calories: 305,
        fat: 3.7,
        carbs: 67,
        protein: 4.3,
      },
      {
        name: 'Gingerbread',
        calories: 356,
        fat: 16.0,
        carbs: 49,
        protein: 3.9,
      },
    ]

    return {
      headers,
      desserts,
    }
  },
}
</script>