Shell: rename prechecked to validated and remove applied

Context

Remove instances of the old term prechecked (now validated) from the mempool in lib_shell, and remove the obsolete applied classification. Also rename the applied field of mempool RPCs to validated internally in the codebase (external calls are unchanged: see follow-up issue #5720 (closed)).

This MR does not change any semantics.

Fixes #4448 (closed).

The term validated has been chosen rather than valid because valid is very difficult to search for, as it appears in a lot of other terms such as prevalidator, validation, invalid etc.

Also do some light clean-up in the first and last commits.

Manually testing the MR

CI

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• n/a Document any change to the user interface, including configuration parameters (see node configuration)
• n/a (no semantic changes) 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).
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• Select as Assignee the next person who should take action on that MR