[DataGrid] Add showToolbar prop to enable default toolbar

[KenanYusuf]

Part of #15823

Changelog

Breaking changes

• The showToolbar prop is now required to display the toolbar.

  It is no longer necessary to pass GridToolbar as a slot to display the default toolbar.

   <DataGrid
  +  showToolbar
  -  slots={{
  -    toolbar: GridToolbar,
  -  }}
   />

[mui-bot]

Deploy preview: https://deploy-preview-16687--material-ui-x.netlify.app/

Updated pages:

• docs/data/common-concepts/custom-components/custom-components.md
• docs/data/data-grid/column-visibility/column-visibility.md
• docs/data/data-grid/components/components.md
• docs/data/migration/migration-data-grid-v7/migration-data-grid-v7.md

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