Replace recommendations/samples using md5 and hash with xxhash

[mattlord]

It leaves general references in place as they are still valid vindex types. It also leaves references to the examples unchanged as the examples have not been updated to use xxhash based vindexes yet.

This is a follow-up to: https://github.com/vitessio/vitess/pull/16113/files

Changes pages previews:

• https://deploy-preview-1772--vitess.netlify.app/docs/21.0/user-guides/configuration-advanced/createlookupvindex/
• https://deploy-preview-1772--vitess.netlify.app/docs/21.0/user-guides/configuration-advanced/resharding/
• https://deploy-preview-1772--vitess.netlify.app/docs/21.0/user-guides/sql/vtexplain/
• https://deploy-preview-1772--vitess.netlify.app/docs/21.0/user-guides/vschema-guide/sharded/

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