RPC doc: deprecation policy

What

Defined RPC policy and set Sunset dates

Historical context: !1982 (closed)

Why

We need to define a way to remove the deprecated RPC endpoints.

How

Updated RPC docs.

Manually testing the MR

Open artefacts and check the doc:

1. Pipeline -> doc -> documentation:build_all -> artefacts -> browse -> docs -> _build -> index.html
2. In browser: introduction/versioning.html

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR