[Docs] Saved Objects API deprecation notice

[TinaHeiligers]

The Saved Objects REST APIs have been deprecated in preparation for preferred versioned APIs. While these APIs have always been experimental and subject to change (or be removed) at any point, we realize they've been available for a while and most likely used by external consumers.

To give consumers enough time to migrate off of these, we're planning on deprecating them within an 18-month time frame.

Internal consumers will be working on implementing replacements but we need to determine the extent to which these API's are used outside of the context of Kibana source code.

As such, we need to update the docs to reflect their deprecated state and add alternative API's as they become public. At the moment, there's only a subset of alternatives for managing Data Views.

Note: The API's for import, export, resolve import errors, and rotate encryption key APIs are not deprecated and are not to be removed in the immediate future.

[nicholas-r-king]

When is this the removal occurring and what API is going to replace the Find Objects API for searching and acquiring object ids/objects. We have a use-case that requires this API and we don't want to build ourselves into a hole if this is still occurring without a good replacement.

[matthiasledergerber]

Same for us.

[rishithaminol]

Still I couldn't find a replacement API for this in the documentation.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[TinaHeiligers]

The date of removal will take a deprecation period into account.