DAL/Node: remove the sqlite3-backend experimental feature flag

Fixes #7527 (closed)

What

Remove --sqlite3-backend experimental CLI flag and configuration option since sqlite is now the default for DAL store version >=2 and >= v22 releases use DAL store version >=2 .

Why

Because SQLite is now the default storage backend for managing skip list cells.

How

• Remove --sqlite3-backend experimental CLI flag and configuration option
• Add an entry in the changelog

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