Shell_metrics: remove applied and rename prechecked to validated

Context

Following-up on !8857 (merged), this MR renames the octez_mempool_pending_prechecked metric to octez_mempool_pending_validated, and removes octez_mempool_pending_applied.

Related: nomadic-labs/grafazos#9 (closed), nomadic-labs/grafazos!38 (merged)

Manually testing the MR

Checklist

• n/a 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 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