v22 : DAL/Node: deprecate the experimental feature flag `sqlite3-backend`

What

Deprecate the sqlite-backend experimental feature.

The flag has been freshly removed in master and we don't want to apply these changes in the code with the final v22 release be next week. So we just want to deprecate the flag for now.

Why

How

Manually testing the MR

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