[DOCS] Remove 'analyzed string' references (#51946)

The `string` field datatype was replaced by the `text` and `keyword`
field datatypes in [5.0][0].

This removes several outdated references to 'analyzed string' fields.

[0]:https://www.elastic.co/guide/en/elasticsearch/reference/5.0/breaking_50_mapping_changes.html#_string_fields_replaced_by_textkeyword_fields