Document that the Jina dense embedding model will become the default model for inference endpoints used with semantic_text

[kosabogi]

Starting from 9.4, and on Serverless earlier, Jina will become the new default model for inference endpoints used with semantic_text. This change will have broad impact across the documentation.

Affected pages:

https://www.elastic.co/docs/solutions/search/semantic-search/semantic-search-semantic-text
https://www.elastic.co/docs/explore-analyze/elastic-inference/connect-self-managed-cluster-to-eis#create-an-index-with-a-semantic_text-field
https://www.elastic.co/docs/solutions/search/get-started/semantic-search#create-a-vector-database
https://www.elastic.co/docs/reference/elasticsearch/mapping-reference/semantic-text-setup-configuration#default-and-preconfigured-endpoints
https://www.elastic.co/docs/reference/query-languages/query-dsl/query-dsl-semantic-query#inference-endpoint-selection

Related dev issue: https://github.com/elastic/search-team/issues/12379?reload=1